I have 5 brothers. I know the smell.
My mom ended up taking out a lot of fabric from their rooms, because the fabric holds the smell for a long time. No curtains, no rugs. Just their bedding, which got completely washed every week. (I mean, the smell clung to the bedding to the point where I insisted on my own sheets, because even after washing, sometimes the smell would still be there, faint, but there.)
Open the windows every single day for half an hour. Make him pull everything out from under the bed and wash everything washable. Have him check, under your supervision, for food left in odd places. Make sure towels do not sit in his room and mold away, but are hung up to dry in a room with some ventilation.
They make laundry detergents now that are formulated to remove the smell from stinky clothes.
No lie, I once got so fed up with all the once-worn, stinky gym clothes the boys had hung up in the bathroom to dry and the 7 pairs of stinky sneakers also hanging out in the bathroom ("Put them in our rooms? But they stink!") and shoved them all under one of the boy's beds. Took them two days to find them, because the room was already so stinky that even all that didn't make a difference.