Can we just be FWB, or could there be an emotional attachment for him or me?

It's not worth the risk if there are any other people who could potentially be harmed by your FWB affair. If either of you have spouses, partners or children who could find their lives upended by your desire for sex-without-emotion, then it's not fair to risk their happiness or family life for a quick romp with a co-worker. That would be supremely selfish on your part.