Well he's breaking the law if he dates you, first of all. Secondly, if you guys do end up having sex that is rape, regardless of whether you consented or not.
Sorry, but for a 20 yr old to want to date a 15 year old, is a little off.
I say find someone else your age, or just wiat it out with him(the 20yo).
If you want to be with your ex, then only yuou know what your heart feels. You're young, enjoy life and don't rush things