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’m getting married to the most wonderful person I know on Monday. Today we got the marriage license!