I loved Magic Tree House at his age! That’s exactly what I was thinking—MTH is wonderful for escapism. If you can get him onto something that exercises his mind, that’s the ideal, hence my drawing, reading, and chess suggestions—anything he can sustain in isolation and when low on funds or access to shops.
Edit: magic tricks if he’s dexterous!
Unfortunately his mother is still his guardian, so she may very well one day pick up and leave with him. In that situation, at least he’ll have the skills to keep growing his interest(s) and entertain himself.
I’m getting married to the most wonderful person I know on Monday. Today we got the marriage license!