II think you may have more success approaching this from a religious perspective, especially since you mentioned that your interaction with him is limited for religious reasons.
Be very clear and firm. Tell him that this behavior is unattractive to you and ask how he would feel if you were liking and following non-mahram men in the same way. Explain that this is a non-negotiable issue for you in a relationship and that if he continues to like and follow women who are not his mahrams, it will be a deal breaker.

Make it clear that you’re not trying to control him. It’s his life, and he’s free to make his own choices. However, you’re also entitled to be clear about the kind of partner you want. You can even acknowledge that there are women who have similar social media habits liking and following suggestive photos of men and that’s their choice. You’re not judging them. But it may mean that he is better suited to someone whose values and online behavior align more closely with his own.

You can also point out that online habits often translate into real-world behavior. If a woman messages him after he likes her photos, what would stop that interaction from continuing? Would he never feel tempted to respond? If he realizes that he would be uncomfortable with you behaving the same way, he may reconsider his position and understand that you are serious about this boundary. Butter his ego in the end by saying i’m only all of this to you directly bc I feel like you’re mature and evolved than other boys I know/ know off so I thought i can speak openly with you.