He sounds like a serial cheater...Not someone who is poly or wants to be poly.
He...not you, has to find the root of why he is non-monogamous. Is it the thrill off "cheating", is it sex...is he an addict in some way.
Honesty...comes with yourself before it can come with others. If he isn't honest with himself he can't be with you.
Anyways, lots of red flags, I am sure you have thought about that... The best way to approach it, is honestly.
"i want to be involved with you in an open relationship but I want to ensure it is open and honest." might be a good start.