Reason #467 why I hate FWB. Which is identical to the previous 466 reasons - it's that, inevitably, the parties stop being on the same page. This creates a degree of stress that is utterly unnecessary in life.
When you progressed to dating, did you have the exclusivity talk, or not? Did anyone set out how they thought things would change, and how they would stay the same, with the changeover?
Contact him and have a conversation this time. Here's a good opener: "What are we?"
If you are exclusive bf/gf, then him dating others is problematic. If you are bf/gf but not exclusive, then it's not a big deal. If you are FWB, then there is no expectation of exclusivity although a responsible person should take care not to introduce STDs into the equation. If you are friends, then you're friends - but you also don't sleep together. And if you're not friends anymore, then feel free to hang up and delete his contact info.