If the issue is at all linked to autism (and don't panic at the thought of that possibility - it can be GOOD news) then you could well be observing your child's behaviour having been modelled (possibly inexpertly) on your own behaviour toward him. These kids often treat everybody the way they are treated. So if you are a teacher who uses sarcasm, for example, you will find your Pervasive Developmental Disorder (PDD) student trying (and possibly failing) to use the same techniques back. So if you tend to shout at your kids, they will shout back at you (often using the same phrases). If you use pressure and choices, they will do the same to you. When your child says to you, "If you're a good Mummy, I will go to bed tonnight when I am told," you can hear the child using your own phrases.
Pervasive Developmental Disorder (PDD) kids especially do this, because they are really bad at modelling their behaviour on other kids. They don't learn by being told, so much as by observing. So you can say to the child, "Don't hit other children, it is wrong to do so," but if the child himself experiences other kids hitting him (especially if they get away with it) then what the child actually LEARNS is not "Don't hit other children, it is wrong," but instead, "Other children can hit me, that is socially acceptable. But if I hit them back I will be caught and get into trouble."
These kids may seem very bright and so people often don't credit these difficulties. But there is a big difference between a bright child who can recite the alphabet at 2 and who can wire up an electric circuit at 4 years old, and the child who just doesn't understand that when the teacher says it's time to sit down and listen, that it is what he has to try to do.
A Pervasive Developmental Disorder (PDD) kid (for example) will need to have the right behaviour modelled for him as well as be frequently rehearsed with it. He also needs to NOT have the WRONG behaviour modelled for him because tat confuses him. Unfortunately, we are generally surrounded by all sorts of behaviours.
A child who models his behaviour on the parent will resemble a miniature authority figure and also will not necessarily understand the 'place' of child vs adult. That's when you hear the child say, hands on hips, "Because I said so!"
They see that this works for you; so why shouldn't it work for them?
I don't know why your son took the card from the birthday boy and threw it in the puddle. Maybe he was trying to instigate a game. Maybe he wanted the boy's attention. Maybe he was frustrated at the boy being the centre of attention, and all that attention seemed to connect to the presence of those birthday cards. What did your son say when you asked him? What was his mood when he did this? Before? After?
Keep this sort of thing diarised, it is important both now, and in years to come. It also helps in years to come, for you to realise how far he has come. You need it to buck up your spirits.
Sorry you need us, but glad you are here. Welcome.
Marg