Straight Crush & Flirting

A little bit of backstory first, I'm 17, in college and I'm not openly out yet. Anyways, I have this friend who I hang out with in school and play for the same rugby team. About 6 months ago he started flirting with me as a joke so I went along with it (I liked the attention to be honest) but recently he's started doing it more regularly (he'll run and hug me, pretend to kiss me, etc) and I don't know if he's trying to tell me something or doing it as a joke - since there's pictures of him with a few girls on his Instagram. The annoying part is I really like him and I want to tell him how I feel but I don't want to ruin our friendship if he's joking. Any advice would be appreciated :)TL;DR: I'm gay, have a crush on friend. Said friend is straight (I think) and being flirty don't know if he's joking or serious and don't know if to tell him I like him or not.