Rumors like these quickly become deeply hurtful, and can unfairly tarnish opinions about people who are not present to participate. It's fine to 'wonder' about another person's manners, behavior, or choices that differ from our own, but until we actually ask that person, there's no way to know what's going on with them.
Speculation is unpleasantly popular in the media, but other than to give us a starting point for finding out a more complete truth, especially in a complicated situation like you describe, it's not useful and quickly devolves into gossip and possibly faulty judgments. We'd be a more compassionate species if we did less of that, and I'd talk to my children if they were participating in the murmuring. How would your son feel if neighbors were talking about him in a similar tone? We don't often question our own thought processes.
I think if I were in your position, caring as you do about this boy, I'd take a sympathy card, and a casserole or some flowers, to the aunt, ask how I could help, and leave it at that until another helpful step forward becomes clear. (You may have done this already.) The mother could be anything, perhaps a woman unable to connect with her own child for psychological reasons. Or she could be deeply grieving and unable or unwilling to face well-meaning strangers. I believe I'd be in that group. And as far as raising money, that would be up to caring friends to organize. I would personally be far too uncomfortable to set that up for anyone in my immediate family. I think lots of people would find that inappropriate.
If you feel the boy's memory "deserves more," you and other caring neighbors are free to hold a less-formal memorial of your own. This might be helpful to the children in the neighborhood or school who knew this boy. I'm sending up a prayer for all concerned. Kindness!