We've been down this road. Be aware, part of the problem could be a physical issue. My kids would spend more effort trying to find an answer for the problem with the least number of pen strokes, rather than a full answer.
You need to put some rules in place, especially as kids get older and are expected to answer more fully. I remember we got drilled on this when I was at school - you always answer in full sentences. So if the question is, "In this passage, why does the sun shine?" then the answer must begin with, "The sun shines because..." I used to begin writing my answer, then go search for the meat of the answer to fill in the blank. I used to find it annoying, but whenever I was reading over my answers months later (for study) they made sense. Otherwise you have to read the questions again too, to make sense of it all. So the first and most important rule of answering - answer in full, with the part of the question you are answering, becoming the first part of your answer.
Your son may need it made that clear and that specific.
Get your son to read back over an older piece of work and see how easy (or not) it is to make sense of it. Then you do one the right way, and show him how it makes it a lot easier. Point out that it is also a courtesy - the teacher has to mark this work, and the students who make the task easier for him (by using full sentences) will be getting better marks. And it's not just for the teacher - if he has to write a report when he is an adult in the workforce, then this is where he begins to get into practice.
Some examples of good answers:
"The sun shone because it was daytime.
The wind wanted a bet with the sun to see who was strongest.
The wind blew.
The man held his coat tighter.
The sun shone.
The man took off his coat."
You can see the questions implied in the answer. But what is more, the answers also tell fragments of the story (if you haven't guessed, I used Aesop's fable "Persuasion is stronger than force").
Contrast with your son's likely responses:
"day.
for a bet.
blew.
held tighter.
shone.
took it off."
You can see the difference and why this sort of answer, while technically answering the question, is only relevant if it is in immediate vicinity to the question. If you are in a conversation and someone says, "Where are you going?" and you answer, "the mall," your answer is a shortened version of "I am going to the mall." The short answer is only valid, if it immediately follows the question. And for the marker, some time has elapsed since he wrote those questions, to when he gets the answers. Hw should not have to keep checking back to see what the answer relates to.
You can still answer a question in full this way and keep it concise. Brevity is to be valued, I am with your son in this. But a single word answer is not brevity, it is rudeness (even unintended) and it is ambiguous.
A sentence has to contain a verb and a noun. A subject and predicate. Even a sentence as short as "Wait!" contains a verb and noun, subject and predicate. Because it is itself an abbreviation for "(you) wait!" Subject is "you" (implied).
So nag him about sentence structure. Also nag the teacher to insist on sentence structure specifically. He's not making himself sufficiently clear.
Other than that, his teacher needs to model the answers for him and also explain why.
Marg