It's possible that the nightmares could be connected to a change in his mental state, but it's a very complex issue. Vivid dreams (including nightmares) are basically your brain trying to sort out more information than it usually has to deal with. We remember our dreams more when we're more wakeful. This can happen if the dream is making us concentrate more, if the dream is disturbing our sleep in other ways (as in nightmares) or if something else is disturbing our sleep (all sorts of reasons, including neighbours making noise).
Our dreams are affected by sensory input as we sleep, also. Plus they're fuelled by the amount of backlog our brain has to get through. If we've had a really busy and challenging time over the previous day(s), then our dreams will be more vivid and coloured by recent events. They are our brain's response in trying to catalogue it all.
The images we dream, are symbols which mean something to us. ASometimes the relevance is just because it's topical, but there are some symbols which have a fairly universal meaning. Death, for example, doesn't mean death necessarily, it usually means change. When we dream of a house, especially if in the dream we sense it is where we live, then the house we are dreaming of represents yourself, your own mind. So if you dream you are in a messy, untidy house and can't find anything, it is the dream telling you NOT that you need to tidy your room, but that your mind is a whirl of confused thoughts that are stopping you making any progress in your life.
Of course this will be aggravated if your own thoughts are confused, or highly variable, or in any other way influenced by your mental health.
Your son dreamed that the Grim Reaper was trying to make him and his dog do something bad. The Grim Reaper could have been connected to Halloween symbolism, or it could have been his subconscious using the proximity to Halloween as a reminder to consider where he is in his life at the moment, in terms of how he is dealing with change, or personal responsibilities. His puppy was involved - "man's best friend". He wanted to keep his dog safe form doing the wrong thing also, so he was feeling a sense of responsibility to a younger, more vulnerable individual. The puppy could even represent himself when younger, sort of the more vulnerable child within. He didn't want to do bad things (which would hurt people) so he hid, to get away from it. Part of the symbolism could involve change (or fear of change, or changes in him making the negative thoughts wose for him, harder to resist) but in his dream, he resisted. In a way, his dream was a rehearsal, a safe way to practice skills of self-discipline.
Something in his days, at the time of the dream, was stressing him to the point where these thoughts were becoming a bit more prominent, but he was resisting them and reporting them. His brain challenged him and helped him review the information and try to develop better mental strategies for coping.
Sometimes a nightmare needn't have a bad meaning, sometimes it's just our brain trying to grab us by the neurons and say, "Pay attention! This is important!"
I find I get more vivid dreams and often more nigtmares, when we're away from home on holidays. The more strange things we see, the more new experiences we have, the more we pack our days full of fun and adventure, the more vivid the dreams/nightmares. If I've been worrying about how tihngs are going at home, those fears can become reality in my nightmares; all that is happening, is my mind is trying to say, "What is the worst that can happen? Face it, so you can practice how you would deal with it. Don't worry, it will be OK."
If you can, encourage him to tell you about his nightmares (or vivid dreams) as soon as possible after he has them. If he can write it down it is also a useful tool. If as he is telling you, or writing it, a symbol seems to feel familiar, encourage him to try to identify the 'layers' in the symbol, so any further meaning to it can be linked to the dream. It makes a big difference with understanding.
Example: My final exams in high school (equivalent to college finals) was a very high-level Maths paper. I had to study really hard for it, it was a difficult paper. But it was the last - then we were free! That evening my boyfriend's parents took us out for dinner to a steakhouse. I wasn't used to eating steak and it was the first time I'd ever had a side salad. I enjoyed the night but I had been nervous - these were society people and I was definitely from the wrong side of the tracks (literally, in this case).
That night I had something in between a nightmare and vivid dream - salad bowls were floating in front of me, in an array grid sometimes 2 x 3, sometimes 3 x 3, sometimes 3 x 3 x 3. There were different ingredients in the salad, they were trying to multiply themselves in matrix form and the resultant salad was dependent on what went in at the beginning of the matrix formation. Bizarre. And I had a really lousy night, because my mind was concentrating really hard to get the right answer.
No, that dream didn't really mean anything important. It's just that all my hard study, plus period of intense concentration both during the exam and then later my intense concentration at dinner to show good table manners and not seem to be too much of a social failure, overflowed into a dream as my brain tried to catalogue everything. My brain wasn't showing any discriminating intelligence, it was simply cataloguing because I had during the day put in a period of intense mental effort, which usually in life indicates something that was important to the survival of the individual. If I had been a cave man being chased by a sabre-toothed tiger earlier in the day, followed by a fight with my wife, I might have dreamt that night that my wife was a sabre-toothed tiger trying to get into the cave. Who knows?
Your son's dreams are worth mentioning to the psychiatrist, but don't be surprised if the psychiatrist doesn't think it's very important.
Oh, one more thing - some medications can have quite an impact on your sleep patterns and on dreams. I was taking moclobemide years ago, and had a rough time with it. It made me depressed and my dreams were not only really vivid, but seemed so real that on waking I had difficulty for a while distinguishing between the memory of the dream, and reality. The images were often very distressing at a very sophisticated level, as if my brain were trying to find the most exquisitely effective ways to torture me. One dream was of my operating meticulously on a rabbit (or some animal) and feelnig unaccoutably sad while I did so, because I had to do my job despite my views on vivisection. The animal had to die at the end of the operation. Then in the dream I stood up and stepped back, and realised it was my own baby daughter.
That was when I went to the doctor and said, "Take me off those pills."
I hope I haven't confused you, but this is a complex topic and there are no easy answers. It is so often dependent on the individual's own subconscious.
Marg
