I'm thinking you wait until he ends his current relationship and has time to process the breakup before dating this person...as you mentioned you don't want to be considered cheating and you probably don't want to be a rebound.

If he is giving mixed signals, just make it clear that since he is in a relationship you have no desire to date him...and even then I'm not sure I would suggest dating him if he is unable to stop his flirting with you while he is dating someone else...you may end up being that person he is dating and he flirting with another girl...