Don’t know what to do about my son

My son (17) use to use an old phone of mine until we got him a new one (my old phone he was using was an iPhone, and he now has his own iPhone). Once he got his new phone, he gave my old one back to me. I had put it away in a drawer and forgot about it for months- a few months back I was cleaning out my closet and stumbled across my old phone that my son had used. I decided to turn it on to see if I had any old photos in it. After it had turned on for a while, I realized that my son’s text (current) were still coming through to the phone. I am guessing his iCloud account it still logged into the phone? Because only “iMessages” come through. Anyways, I couldn’t help myself and I took a look through his texts, I ended up in his text thread with his girlfriend (17F) .. I know this is a complete invasion of privacy and I am not proud of it. But, as I was reading their conversations, to a complete surprise I found out that my son is extremely mentally/verbally abusive, controlling and extremely jealous- calling her (slut, whore, etc) and he is completely gas lighting her. I am at a loss I don’t know what to do and I can not believe my son behaves this way, he was not raised around ANY type of behavior like this at all! I have also noticed he lies A LOT ( compulsive, pathological) this is not doctor diagnosed- only my thoughts! I want to get him into therapy I just don’t know if it’s too late. I also do not know what to do about how he is treating his girlfriend.. she is a very sweet/kind girl. Do I tell her mother about what I found? I know my son would be devastated if I did this as he would likely never be able to see her again, and it would likely cause problems with school etc. I guess I am asking for what you would do if this was your son?

No negative comments please. Only looking for friendly advice.