This is a tough one, so many factors. But the young man is not a child, he's a young adult. Sounds as though he could benefit from bereavement counseling (groups available in almost all areas), as well as needing to seek recovery for substance abuse.
As far as finding drugs (or a dealer) at an NA meeting--drugs can be found anywhere for those who wish to use. Whether its NA or one of the secular groups--that kind of support is crucial.
Is your husband a recovering addict? Are you ready/willing/able to turn your home into a sober living environment and all that entails? Written contracts, clear, firm, enforced boundaries, etc.
It's understandable that you want to help in this situation, but if you're naive or ill-informed, all of you could be in for disaster.
You've made a wise decision posting on this forum. Lots of experience and information here.