What happened to him in the army?

That's a question only he can answer. I knew a guy that completely broke down after he found out his wife cheated on him, he had to seek psychiatric help and everything. Any number of things could have happened to him that would change him, including but not limited to what he may have experienced in Afghanistan or Iraq (although those are the two most likely cases of his apparent personality change, don't automatically assume that they are the cases). There are a lot of sources for vets that might have PTSD or just need help in general. If he looks like he needs help, be a friend and get him help.