That makes it a little more tricky.
If you were setting up part of your basement, a back room, or your garage, that would make it easier, but more expensive.
Most of what you can do for something like a bedroom is going to be to try to control outside noise from getting in, and adding sound absorbing material to help keep the sound from bouncing around too much.
I had a book (once upon a time) that showed how you can make living spaces into decent quality recording spaces. I'll see if I can find it.
They used things like couch cushions and mattresses to convert a living room into a temporary studio. (Sounds hinky, but the author was a sound engineer and a musician.)
My bet is that you could buy some 1/2" plywood panels and 4" foam from an upholstery shop and wrap them with blankets. Make them to fit the door and windows, and a couple to put in front of the middle of large plaster walls, and you'll probably improve recording quality quite a lot.
Then he can store them someplace else when he's not playing.
You should be able to get all of the stuff for a couple 2 or 3 hundred bucks, and he can take it with him anywhere.
