G.B.
I'd simply invite his friends and classmates over and have refreshments for them. They won't want any theme or decorations. That is juvenile.
You could do just his best friends too and do something like get him that video games he's really been wanting. Something they can all take part in.
I'd keep it as simple and non kid like as possible. Maybe even just a pizza party with no other activities. They'll just sit around and talk but still get to eat. That's what teen boys like most.