How do I tell my call/friends w/benefit that we can no longer do what we do?

Ok, so I met this guy about 4 years ago and we started dating. We were always together going out, hooking up and just having really good times. I started caring for him dearly, but the way he acted (never disrespectful) gave me the impression that he wasn't looking for anything serious. So years went by and we sort of just became friends w/benefits. I've talked to other guys before, but I met someone really special that loves me a lot and does everything in his power to make me happy. I'm 26 and at this age it's hard to find that kind of love now days. I'm now in a serious relationship, but I still talk to myFEBB( no, we haven't "hooked" up in like over a 2 months) The other night my FWB called me and wanted to kick it. I lied and said I had to look after my god kids. How do I tell him we can't hook up anymore without it jeopardizing our friendship?