It might be a bit easier as he gets older. My DS's friends (aged 6-7) run beside him in his wheelchair, they play frisbee and the other kids will often offer to collect his throw and return it to him, they offer to push him (which I politely say no to).
Does your DS have a couple of friends he is closer to? If so, I would invite them over for play dates. Children are pretty good at adapting to their friend's needs. If my DS is having a period where he's too tired to leave the house, one or two of his friends will often come around and uncomplainingly play Monopoly or do some art with him. (I suspect their parents may have explained in advance that he's not well enough to be active at that time, so expectations have been managed in advance.)
Parties are harder, but so far the parents involved send a text or call to say, 'this is what's going to happen at the party' (e.g. trampolining, slime making, climbing etc). So far they've all been very welcoming and said DS is very welcome to come, regardless of how much he is able to participate. I just decline the invites which are for very physical things, like being held at a trampoline park. We've been lucky and most have been parties where DS can fully participate, like slime parties.
I would consider being referred to a community physiotherapist, as they often have access to soft play areas that are accessible. Also look for charities that support children with differing needs as, again, they often have access to particular spaces, arrange days out to places that can seem difficult to get to when you're on your own and organise big summer parties and sports days etc.
Your little one might be a little young, but it might also be worth looking into things like wheelchair basketball groups?