I'm just like, what did you expect man? Anyone's gonna be super sus if a bag full of money just appears in their house. I'd assume it was stolen myself, or otherwise involved in some sort of shady illegal business and I wouldn't touch it with a 10 foot pole lol.
Honestly what he should have done was like, tell his mom that he was friends with her son and they put some money in together in bitcoin or stocks or whatever. He then struck it rich after her son had died and he felt like he should give some of the profits to her since it was partially her son's money anyway. She probs wouldn't have accepted the whole amount, but would have at least been way less sketch than just anonymously dumping a million bucks on her door step, and she might have accepted some of it and lived well. Especially if he really played up on the "your son would have wanted you to have it" card.