I think it's tricky. Large all-class parties are actually not a great time to establish friendships between children, or even put faces to names: it will be a chaotic, noisy few hours, and then you'll be mopping up and it's possible nothing much will have changed. You may find most of the parents will drop off (especially if you have entertainment) so there's little more than the same hello/goodbye dynamic, not a chance to get to know anyone.
I would concentrate on establishing relationships with a few other families, getting someone over for a playdate or arranging a meet-up in a park, or offering to share pick-up for clubs. It may be quite stilted and awkward, but it's a start. At that age, friendship groups have formed but they're still very flexible. Is there a parent WhatsApp group?
I would also expect the school to help manage a new pupil's social transition, so it might be worth having a word with the teacher, especially if DS has had negative experiences elsewhere.
If your son would really enjoy a big party (and they're overwhelming for some kids) and you don't mind organising one, then I'm sure it would be fun. But keep expectations low. Children that age seem to enjoy parties then immediately forget them. It's not like going on a big night out with colleagues, and bonding, and everyone talking about it afterwards. DD (also year 2) recently went to a party where the birthday child didn't even attend, due to sudden non-serious-but-unfortunately-timed illness, and they all just partied on regardless and sang happy birthday to his sibling. They're not callous kids: it was just another weekend, another party, and they all know the drill.