Everyone is different, all of us family members and all individuals suffering from these illnesses.
In that situation, this is what I would do for my specific situation:
1.) I would try to establish contact and communication via e-mail (library has it for free), maybe a PO BOX I rent for my family member, also a phone card I photocopy (and send over scan of it in e-mail attachment) and pay for or maybe find out a phone number at a shelter where he stays. I would not send money because my family member has been robbed and has also gotten drunk sometimes.
2.) I would definitely look into local resources as szadmin suggested. If I knew where he was I’d call everyone I possibly could, write letters, try to find a way to provide resources, get him hooked up with services, food (figure out a way to buy food for him?), clothing.
3.) I’d offer a bus or plane ticket home and invite my family member to live with me. (Not an option for everyone)
4.) I’d probably go looking for my family member IF I had the resources and would not lose my job, etc.
Here are some steps you can take from where you are if you can’t locate him:
http://www.outpostforhope.org/looking-for-a-lost-loved-one.html This link has a pdf guide you can download with steps to take to locate your son if he is missing.
Even if you wanted to force your son to do something, you couldn’t. I hope he feels better and has a long life with recovery in a way that he chooses. I hope you find the supports and resources you need.