It doesn’t need to be expensive. If cost is an issue, then food would be the priority. It’s hard to explain how hungry my 15yo ds is.
WRT activities, my instinct is you’re just doing stuff to get him out the house so he can properly being in the house again 😁 So driving to the supermarket is one pp have mentioned, even if you usually get food delivered. And don’t ask if he wants to go the supermarket, just say ‘right, let’s to Tesco and get some food in’. He may well look disinterested and surprised, may play on phone for most of car journey, but is actually quite happy. Teen boys can seem so grown up, but they are still quite young and they get a lot of security from adults in their life being calm and confident.
I think a couple of bigger trips might be nice if you can afford it, but you don’t have to spend too much money. Again, don’t second guess yourself about whether he’s enjoying these trips. Just accept him wherever he’s at. Loads of times you think a teen boy didn’t like something at the time but then they chat about it afterwards with real fondness.
And obviously let him know to tell you if there’s anything he needs or fancies doing. That’s a better way to phrase it that than asking him what he wants to do. Or give him two options.
Do let him set up his games console ASAP. It’ll be how he keeps in touch with friends.
Good luck. You sound like an amazing aunt.
One final thing. I have two teens - a boy and a girl. They both can have huge hormonal mood swings, but it’s easier for dd cause they’re linked to her menstrual cycle so she knows what’s happening. Ds can have real dips and they feel so out of the blue to him. He’s had a tough couple of weeks, but now he’s okay again and Dh pointed out to me that he’s suddenly shot up half an inch. So the hormones are real, and they’re usually thinking about themselves nor you.