Since you have a limited budget, which many of us do, even $200 can be a lot, and especially since your son's B-day is in December... when everyone will have budgets to consider in light of the Holidays.
For us, my daughter just had her B-day party at Chuck E Cheese... it's a good package deal. But, even for us, the costs were unaffordable.. and luckily, my daughter's Aunty "treated" her for the party and paid for it. She is a really nice and generous Aunty who has no kids of her own.. so this was her way of "helping" us with the expenses. Sometimes, Grandma will pitch in too.... mind you, we don't "ask" them to do this.. but they are just very generous.
Perhaps, have a party at a public park... and maybe make it a potluck?
Or, if one of your family members, who may have a bigger yard space... can "volunteer" for having the party at their place?
For party goodies... www.orientaltrading.com has LOTS of party things, for good prices, in bulk.
And for activities, have several balls... since kids end up always playing with balls in all kinds of ways... and then maybe have a fun "team" type games... or geared down for kids 'adult' type games... or egg on a spoon games running relay games etc. Relay games or any type of running games are always fun for kids AND adults.
Good luck!
Susan