First, does he know and acknowledge he needs help?
My brother got out of the Army almost 2 years ago and he has PTSD as well. He just now took a full-time job because he couldn't stand to be around people. If a storm hit in the middle of the night he'd yell and scream for everyone to take cover :(. It cost him a lot of relationships because other people really don't understand how to deal with it. He's my brother and I'm going no where, but a lot of others really just moved on.
My parents and everyone in our family has pushed him to get help. The VA has been less than helpful to him. I'm sure there are more resources he could look up (like some of the foundations mentioned in previous responses), but he doesn't think he needs it. My husband supports the Wounded Warrior Clinic at Bethesda Naval Medical Hospital and he ahs tried to get my brother to come with him. Because my brother didn't lose a limb, he doesn't see himself as damaged from two long tours at war.. But he does walk with a cane (he's 25) when his back is acting up. His leg will go numb and he'll just fall over. The cane prevents that.
I guess the best thing I can say is do NOT enable that person to continue to be that way. My parents do that by not pushing my brother more, but don't push too much. He has to wait help...he has to know he needs help...until then, I'm not sure what you can do but be there for him.
Good luck. And please thank him for his service and sacrifce for our country!