As others have said, simply providing a stable place to be around another child and adult will do leaps and bounds. But if possible I would work on helping him get into activities he can do on his own when he is dragged to mom’s work and told to sit quietly in a corner or sent to the disinterested aunt—because that’s bound to happen again.
Maybe try discovering if he likes reading and what sort of material? Children’s paperbacks can be found inexpensively; library cards are free either for a physical library or digitally. If you find he’s into reading, an on-sale Kindle could be an option that you regularly load up with Libby ebooks.
How about a small sketchbook and a few pencils if he’s artistically minded? He doesn’t need a whole kit, just a few pieces for him to pass the time. If he’s into games, perhaps teach him solitaire or get him a travel chess set. Maybe sports are more his things, then fantasy football or some such.
I think teaching and helping him develop some low-cost, unobtrusive coping mechanisms could really save his mental health in the long-term. It’s a bit grim having to suggest all these solitary activities for a child, but the reality is that he will find himself in these isolating situations again. Giving him the tools to develop interests and hobbies without his mother/aunt’s intervention will put him in a better psychological place as he ages and will provide naturally-occurring encounters with potential friends who share his interests.
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.