(edit: ah, instagram, gotcha)
it depends. do you talk to him/message him on instagram, or do you only follow him? if you talk to each other, then i don't see any problem with adding him on facebook (and if you're not sure then hey, you can ask him) but if you don't, then I don't really see the point? I only really use facebook for talking to friends though and i wouldn't add someone I didn't talk to, even if we followed each other on social media. it's your call though, worst he can do is decline.