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.
Recently completed HFM Prescott’s The Man on a Donkey, a wonderful piece of historic fiction about the main actors (and a few fictional ones) of the 1536 Pilgrimage of Grace—a rebellion against the religious changes of Henry VIII. Despite being a scholar of 16th century England I’m not at all interested in historical fiction, but this was quite a beautiful work set as a chronicle and tracing half a dozen characters from their youth until the final suppression of the Pilgrimage in summer 1537. Prescott does get straight to business so I can imagine it would be a bit difficult to place oneself without preexisting knowledge of late medieval/early modern England, but that thrown-in-the-deep-end attitude worked for me.
Edit: word is that Hilary Mantel was deeply influenced by Prescott, as was the playwright of A Man for All Seasons.