Having a FWB while dating someone else for a serious relationship?

Sounds like you want an open relationship. That would mean they would also be free to see other people. Unless he's a complete doormat you won't find a guy who's going to be faithful while you are having fun on the side. These sorts of relationships rarely last long though. If things are never going to work out with the FWB you need to cut off the benefits and find a real relationship. He's holding you back, it's not healthy.