Yes, asking him directly what's going on behind his desire for you to end casual sexual encounters is the way to go. But it's on him to figure out what's going on in his noggin and communicate that to you. You should be supportive and a sounding board if he wants that. (It's possible you may be too close to the situation for him to talk openly at first- he may need to have someone else be that sounding board.) But be wary of trying to do too much of his emotional heavy lifting for him. I see many women, often young women, take on this task, often for the men in their lives. It does neither of you any favors in the long term.
Good luck! I think you are approaching this smartly!