I’m sorry for both you and your boyfriend . It’s terribly sad to hear of his assaults and I hope he does pursue counseling. If you want to let him know you are available to support him as a friend with no romantic strings attached, I strongly suggest you first read some of the resources indicated here. This will do two things. First give you a better understanding of what you might be looking at and also give him a little time and space. Then if you are still absolutely certain you can support him with no expectations, write him a very brief note indicating you CARE about him, you are sorry for what happened to him, you are available for emotional support and you have no expectations of having a romantic relationship.
My husband was assaulted as a child so I’m not familiar with the resources or the complexities involved with an assault as an adult. There are a few survivors that provide excellent insights and information from their perspective so I do hope a few of them respond as well. Unfortunately our societal systems and expectations still lead a lot of men to try and bury all of the shit sexual abuse/assault dumps on them, which tends to come out in self destructive behaviors.
I wish you both the best. Know if you proceed to support him and he accepts that support, it can be a verrry long road with lots of highs and lows.