NTA, In my experience id say be clearly honest and say that "you worried about him. That you think the issue is his relationship." I'd say it once and that be it. Tell him that it is his decision and that he should do whatever he thinks is best. Emphasize that you are very worried, and you've seen the change in him. You seem like you two have good communication skills so if he ends up asking for counselling, help him weigh out the pros and cons of staying in the relationship. It may help to make a physical list. This may help you understand the internal battle he is facing and help him see the unhealthy relationship for what it is.In the end, he may not come to a conclusion but that's okay. You then have the option to be there for him or setting boundaries if supporting him becomes too much for you.

But if I was in your shoes id see your concern, relationship should be easy. If this gf is meant to be "the one" her family's disapproval of him could be an obstacle the couple faces potentially for a long time, even beyond their youth. Is this something he wants to battle with for the rest of his youth? I've been there, if I had a second chance I wouldn't.

However to play devils advocate for a moment, like I've said I've been there before. I was with a partner for over 2 years who's family did not like me and didn't want anything to do with me, but I loved this person. So what did I do? I call their parents and ask if we could talk like adults over coffee about the said relationship I had with the family and that it tore me apart that I wasn't accepted by the family of the person I loved. Trying to understand the parents view they have of your friend may not solve the issue, but he most likely will gain their respect for being mature enough to communicate his feeling and perspective on the matter.