Think how much meat per head you'd expect to put on a plate and multiply by amount of guests- maybe a burger and two sausages each plus buns. Triple it. Buy that! One pack of veggie equivalents unless he's certain they all eat meat. Bulk out with precut burger buns and plenty of ketchup/BBQ sauce. A little bit of salad/coleslaw. Leftover meat/buns can be frozen (unlikely to be any though). Cans/individual bottles of fizzy drink or cartons of juice. Bag of ice.
Costco do big celebration cakes, if it's not a birthday then a red velvet would be fab.
He is the host so has to help prep food etc. Stay out of the way if you're not cooking but make it clear that you're in charge. If anyone acts up, your DS has 5 minutes to stop them/evict them or you will do it for him! Hopefully, if you let him know your expectations, he will pass them on to his friends and save himself the embarrassment of you being bossy <can you tell I've had housefuls of teenage boys?!>