depending on the status of your relationship.
I think men like to know they are loved, desired, sexy. They want to feel like their girlfriend couldn't love anyone but them. Stroke their egos. What does he think he's good at? sports, music, sense of humor, etc
Tell him he's funny, tell him he athletic or you find whatever athletic prowess he has sexy... Know what I mean? Whatever interests him show interest in that and support of him doing it
Send him a sexy text (either saying something or a sexy pic).
We aren't that difficult. The easiest way to access our hearts is through our penis (not sex necessarily, but sexual things). The second easiest way is when we feel like the woman respects us and likes or takes interests in our interests