I have 5 year old son as well. He used to be a very reliable kid when it came to playdates. I never had to worry about him doing something bad but around 5.5 he became a pain. All of the sudden we started having problems. He started being bossy and mean if the other child let him. If the other child was in charge all was well though.
I think there were two reasons for it. First of all he needs kids with whom he can play on certain level. I am sorry to say that but some kids are no longer a good match for him. It sounds bad but on the other hand I got to respect that he may prefer not to play with some of his former friends and we stopped setting up playdates with some of them.
The other thing is he his problem with new situations and people. When we meet new kids (as we've done that a lot lately because we are moving towards HS and met a few HG friends) he is nervous and sometimes tries to get attention the wrong way.
It doesn't help that he can get his younger brother on his side and he then starts behaving like a jerk too.
We now have a deal that if he doesn't like a playdate or simply has had enough (been there for too long or is tired for example) he may ask to go home and we will leave within the next 15 minutes. If the playdate is at home he is more than welcome to go to his room and get some alone time if needed.
Things are much better now but I still remember being horrified when my well behaved kid threw toys at other five year old. Gee.
