Parenting

Give him some words to use in stressful situations.

“I need some space”

“I don’t like that”

Especially if he hasn’t been around other kids a lot. They’re loud. They’ll take what you were playing with. They won’t give you the toy you want. They’re in your space. They’ll run into you. Pack 10+ kids in one room with 1-2 adults and there’s bound to be some chaos.

Teach him to ask for help and vocalize what he needs. Sometimes kids won’t advocate for themselves because they’re so used to the adults in their lives knowing them so well that they anticipate their needs.

Also, I second all the independent stuff from the one poster.

Take him to a lil kids park as much as you can. Like every weekend. When you're there, be on the periphery and encourage him to go play with other kids. Don't hover right next. It's ok to make the intros and then go hang on a bench. Idea is to get him playing with other kids by himself, getting into lil disagreements, dealing with kids that aren't nice, etc.

Potty training, make sure he can pull his pants up and down by himself, as well as any coat / jacket. Practice using polite words. Play games that require taking turns. Practice putting on and taking off shoes by himself. Have him pick a calm down method (blowing out birthday candles, etc) and know when to use it.

How old are you, OP?

How young? Potty training is a great thing to get down, and is required by many places.

Spend some time at splash pads and other playground-like places over the summer, that should get some good kid to kid time. (And assuming you have a grown up going as well, you have your own play time)