Hey Hamster.

lol Late to the topic, but I do hope things went well?
He...builds models of ammunition? Like bullets or shells (I would think shells...bullets lack the same...grandeur, I guess?)? I'm generally convinced I am the obligatory forum gun nut, but even then...lol that's a little odd, if not sorta cool at the same time. lol Had I caught you sooner, I could've just mailed you some old .308 brass and made his day.
On a serious note, the balloons sound really sweet.

I would think for your purposes you'd have them inflated in the store, but if you do buy a canister, be REALLY careful with that. If you knock off the valve at the top of the tank, the tank will shoot off like a rocket, with enough force to shoot through a wall. Sort of a guaranteed way to ruin your day.

So do be mindful of that.
That aside, I hope he has a great birthday, and I hope you two have fun.
Hugs,
Harley