I'm not going to outright state that it won't work, but in the vast majority of cases when people who were in a relationship continue to sleep together after breaking up, it results in problems. Even for me, as someone relatively unsentimental about relationships, when I slept with an ex for a couple of months, starting about a month after the relationship ended, it got very weird and it was in the end not worth the confusion it caused.
For all but the most iron-willed of people, having sex with a person you had a romantic relationship with in the past is nigh-on impossible without tangling up old feelings.
I would be insulted if I were you tbh. It's as though he doesn't care that you were in a relationship, like there were no feelings, like it was just sex! I also don't think it's the best idea to sleep with him in a FWB situation if you're not over him. I think this might end in tears if you do.