From what I've read (i.e. NOT A TEACHER/PROFESSIONAL), from about two to three, he might be ready for short bursts of socialising, but it's best if a parent/comfortable figure's still available during this, so he has someone safe to go to if he starts getting stressed. It's also helpful for making sure he's not being bullied (or bullying).
A key factor is that if you're not there, then he's gone from a world where he's the primary focus of attention to a world where he's competing for attention with 30 kids, which takes some adjustment.
Once you're sure that he's comfortable in medium-sized groups, that's when you can think about the right age.
What you really shouldn't do is keep him out of those social events until the age where you have to drop him into pre-school on his own.