You need to tell his girlfriend about his intention. Tell her everything about his douche boyfriend. You need help that girl, because you and her is in same situation which is date with a bastard. This is your time to get revenge and make sure he will never be a part in your life. You are a stupid person if you agree to become his fuck buddy. Use your brain girl, you have it and try to use it. It is better for you to find nice guy and have sex with him rather than with your bastard ex
Ihatelove