The solution is quite simple, give him food, clothes, or other necessities he may need. Additionally speak to a Rav and see if you can get someone to sponsor rehab for him. That would be the greatest chessed you can do for him. Help him without giving him money.
BTGguy- If he is poor you have a chiyuv to help him even if he is a drug addict. There are ways to help him out without allowing it to feed his addiction