If you’re not a 100% sure that he’s seen a psychologist/therapist, the encourage him to go to one. Since you don’t have the training, you’d have to be careful with what you say…because you might (even with good intentions) say something that could hurt him. Seek the help of a professional.
As far as being a friend is concerned…as others above have suggested…you can listen to him (sometimes people only want you to listen…and not advise) and treat him like you would all your other friends. Again, be mindful of your words and body language. For example, people can be offended if your body language reflects pity.
You can try to gently redirect him toward the positive things in his life/his accomplishments. He can’t undo the past but he hasn’t let it consume him completely either…and has in many ways moved forward in his life. Acknowledge his feelings but point out the strength that he has. Again…it’s a sensitive/complicated issue…and what might seem like helpful words to you and us…could backfire with him.