sorry. imo, your son is more important than a hobby. that's imo only. i'm sure he doesn't want to be there either.
for the time being, can you put a wall heater in the attic for the cold weather and then a window ac in the summer. it's a huge inconvenience, i know, but not as bad as moving back home is for him. there goes any adult privacy he ever had.
whatever you do, that's what he'll remember you did. sleeping on the couch like a visitor is not an option. when times are tough, he should feel like he can come to you for help.
EDIT: if you ever need his help, would you like to sleep on the couch?