Trouble in Paradise with a FWB

I have had a fwb for 4 months now. Both of us agreed we didn't desire a relationship with each other (or for anyone else at the moment), we only see each other (we are big about not spreading or contracting stds), we even have an agreement that if we ever "see" someone else we will tell each other (again, because it should be my choice or his if we want to keep having sex knowing that person is sleeping with someone else - A STRICT NO STD policy we hold). The sex is normally fine. He is the biggest guy I have ever been with, but lately I feel as though it may be getting boring for him. He has asked to do anal and I'm not a big fan( I actually prefer not to) but I aim to please him so I said I would try. Any tips ladies? Also I gave a little background because I want to know about your experiences with a fwb and did it end well when it was time to go separate ways? I've become so comfortable with just pleasing him, I don't know if I even want to see anyone else.