Do I let him go or stay and support him as he tries to get off drugs?

This is a tough situation. I think this needs to be a situation of tough love. You need to let him know that it's you or drugs (he can have one or the other but not both) and he best decide quickly. God forbid that he ends up in the same situation you've experienced many family members go through. You've dealt with enough addiction in your life girl (you lost a father and that is more than any girl should have to go through). You AND he do not deserve to go down this path. Let him know that you love him and therefore you are doing this for the good of both of you. If he doesn't discontinue using drugs or you catch him using them behind your back, leave him; and don't look back.