I'm so sorry about your son's alcoholism. I know how you feel, as there are alcoholics in my family, as well. It can tear you apart if you let it.
As you stated, he is the only one who can change his behavior. All you can do is pray for him and refuse to enable him. You should go to Al-Anon meetings, too. I'm not sure, but I would imagine they have them there, or if not, they should have something similar. I've been meaning to go myself. I've heard that it is extremely helpful in learning how to deal with the pain of having someone you love being an addict, regardless of the substance.
I sincerely hope that your son is able to get help for himself before it's too late. He has to be suffering from depression and possibly post traumatic stress disorder (PTSD), too. He really needs to see a therapist, as does everyone who sees combat. I can't imagine the kinds of memories they have to live with on a daily basis. It's no wonder they are so messed up when they come home, and that they turn to substance abuse. Please try to talk him into seeing a therapist. It's worth a try.
I'm hoping and praying for the best for you and your son. Thank you so much for your post and feel free to visit us again.