To do it safely–as in with no risk of endangering the friendship–invent time travel, go back in time, and set up the relationship differently. Otherwise, any move you make is in some way going to risk the friendship.
Which is probably the main reason people find “friends with benefits” to be a misguided fancy more than a workable scenario. That and that one person or the other always wants to “turn it into more.”
There are two possibilities here: either he’s in love with you, in which case all you have to do is ask, or he’s been really enjoying the sex-without-commitment thing, in which case asking will put him in a tight spot that’ll almost certainly result in revoked benefits, friendship, or both.
What you need now is good intel. Drop hints, nudge, pick up some signals, and see if you can get to the bottom of how he feels before pushing further. Ask friends of friends, siblings, etc. If you find out he’s not looking for anything serious right now, you may be out of luck. Sex doesn’t always equal love for a dude, and there’s no helping that.
Or, if you’re impatient like me, just flat out ask him and then cleverly back out of it or pass it off as a weird joke if his face immediately pales and goes blank.