Hi rachibaby,
Plan to test again in a year (basically, when your son has picked up reading proficiently and the learning curve takes off). In the meantime, monitor playdates carefully. I found that there was a period of time my kids did not enjoy other kids, so we just suspended organized play. Instead, I'd read to them and they'd read to me, we'd play board games, bike ride, go to museums. It was way more work for me (and more enjoyable) but I have always found that the more time I spent with each child, the better-behaved they were. This is still true, from my 6 yr old to my almost 14 yr old. It's very demanding! If you don't have that kind of time, see if you can set him up with some independent activities (like legos or computer games) that can be "checked" rather than full-time interaction. Maturity will come with age. Begin a dialog about what is acceptable & unacceptable behavior, using how others feel when he does something inconsiderate ("how do you think John feels when you tease him? Would you like that?") Have consequences for bullying/naughty behavior.
At 5, they're trying to test limits and they need to learn respect for others and empathy. While it's sometimes hard to do, I remember one mom who only moments after she arrived at a playground, turned around and went home with her boys when they misbehaved. I was so surprised! But I've thought about it a lot. She was clearly communicating she was in charge and meant what she said that they needed to behave (her kids were WAY better behaved that mine or most others).
