Personally I am not in to the "invite the whole class" thing - and fortunatelt no-one esle at ds' school seems to be either (he is just going in to Primary 2).
If you invite the whole class now - at the beginning of Recpetion, then yuo are setting a precedent that the other parents may feel obliged to follow, whether or not they can afford it.
If your boy is a "boys' boy", I would go with just inviting the boys, if you really can't get any names out of him.
Or alternatively, cold you go along to school with some semi-completed invititations (ie everything bar the child's names) and get ds to point out who he wants to come.
Ds' own birthday is on Sunday - and although the Scottish schools start in mid August, this meant that last year he hadn't really made any friends. We had got organised beofre the summer holidays and got contact details for his friends from nursery (Scottish equivelanet of pre-school) and invited them, plus a couple fo other friends he has from outside school. Because he hadn't yet made firm friends, it wasn't a big deal. Does your ds still have friends/conact with kids from pre-school/nursery?
This year it is totally different, with the majority being anemd by him for us and are from the school, with a few extra from "outside" school. Interestingly, he has chosen to invite a few girls - one in his class, one the wee sisiter of a boy (who is also invited) in the year above him, who he knows from After School club - and two Primary 7 girls who apparently dote on him!
There are going to be 16 in total - the house wil be in chaos! Fortunately we have a big enough house (amd main room) to accomodate them all. Silly Billy is going to have his work cut out!