He needs to have something he is passionate about. If he joins the Air Force, he would probably get more disciplined, feel better about himself, and go off the pot smoking, since they do random drug tests. But that is only if he is interested in it.
And I disagree that he needs to take care of the depression first. I think if he gets actively involved in a huge career decision/path like this, the exercise, male bonding, purpose in his life, travel, etc, will likely pull him out of the depression.
If he goes on antidepressants, the military probably would not accept him. They don't want depressed people on medication. "I am depressed and don't see a reason for living" should be something that you try exercise, job, etc FIRST before getting on the harmful antidepressants. Studies have shown that antidepressants do not cure depression, plus they all have harmful side effects. What is the main reason a male gets depressed? Because he has no purpose/career to build up his ego.
BTW, my husband is in the military (Army) and he told me that they don't want people on antidepressants. If your brother is truly whacked out, he won't make it through the basic training and they will weed him out. Even though they tell you that once you sign the contract, you are in and can't get out of it, it's not true. If you can't handle the boot camp stuff, you can get out of your contract.