This thread has broken my heart in two. 
for you and your children 
Ok, pulling together what some have previously said and adding to it...
My DD never had kids rocking up to her parties in primary school. NOT because she was super popular, but because she's November born and parents were so full of joy to get rid of their little darlings kids for a couple of hours on a weekend when the weather's crap and it's dark. Given that you're posting this at the beginning of July, I'm assuming the b.day is around now. which is a crap time, even for the kids everyone wants to know.... for the next 3 or 4 weeks, all the extra-curricular clubs with have tournaments and final matches and drama shows and end of season parties, and once those are over, families will start fucking off on their summer hols. So do explain to your DS that even if people really want to come, they can't if they're away in costa by the sea or performing in their end of year show or whatever.
Agree you need a chat with your DS to find out what a "party" actually means to him and what it looks like.
But with my November born (remember - they're coming anyway, even if they hate her!), the key is to provide a party the kids don't want to turn down. Money helps (if you've got money to throw at it, but you've already said you haven't) but it isn't everything. With a bit of imagination you can "sell the sizzle" (as they used to say in the 1980s).
When my strapping 15 yr old was 4, we held a "pyjama party". To the adults, this would have meant some form of sleepover. No, no, no... It was a normal party (2 til 4, or whatever it was) but YOU CAME WEARING YOUR PJs. The kids bloody loved it. They were 4! Cost to me = nothing, although I did buy DD a new nightie for her to wear at the party (for a couple of quid from Asda).
Here's a more age-appropriate example... Organise a treasure hunt (cost = nothing but time) and make it a themed party where they dress up and solve a "murder mystery" to win a "grand prize" (prize = some stickers or something. Cost £1 - 2).
Make the invite something that people won't want to turn down. You can use thought and imagination, and it doesn't have to be expensive. Have a picnic in the local park and call it an "explorers party".
Alternatively, book something pricy-ish, that your DS would love and explain to him that as it's expensive, he can only have a couple of friends, and then spend a day at Harry Potter World or a Theme Park or whatever's cool and travel-to-able from where you live. This also works for the cinema. Limit it to (say) 3, buy all popcorn and rubbish and let them see a film together. Maybe with a sleepover after, if you're feeling brave?
Good luck.
p.s. I have LOTS of party ideas, but they're potentially outing as I've done some of them, so PM me if you think I might be able to help.