I'm sorry to say but that situation sounds terminal.
It sounds very much like someone got his hand caught in the cookie jar, and has vowed never to do it again - but only if he can have his cookie back. If he can't, then he will just keep sneaking some crumbs.
I really don't think you want that or more importantly, deserve that.
Part of what the Bible says about relationships is that husband and wife build each other up. The wife is sacred, precious and cherished as the person most dear to the husband, and the husband is respected, honoured and loved by the wife too. Obviously I know you are not married, but as a Christian I look towards what God has laid out for me. In respects to a relationship, I look towards how God believes husband and wife should act and treat each other and from what you have described, he was taking you for granted and has only realised that because you are no longer there now.
I see nothing wrong with trying to help him, but I would not suggest entangling yourself in that relationship again unless there is significant steps made on his behalf to rebuild the trust.
Corinthians talks about love in the Bible - which is not actually love as we understand it, it's sacrifice. It's about how husband and wife sacrifice of themselves to meet each other's needs. In our culture we often seem to think love is all roses, and if there is fighting, or arguments, then we might not be in love as we thought - but Biblical love is about how you handle those parts of the relationship - real love, true love - gives of itself, to conquer any negative aspects. That's what Christ did for mankind as a whole on the cross, He overthrew sin, through his sacrifice so we had a clear passage back to God.
Only you and your ex boyfriend will obviously be able to sort through this, but certainly I would suggest great caution. I have a friend who was in a similar situation, whereby her boyfriend was always a bit of a ladies man, and was never willing to restrain himself or work on it for her, or to show her respect or reinforce their trust. She didn't act on it, and unfortunately two years later she discovered he had cheated on her several times.
I know you aren't a Christian, but for what it's worth God doesn't want anyone to settle for anything but the very best. Christ's sacrifice wasn't so we could live mediocre lives with cheating partners, constant trust issues and a lot of stress - it was so we could live as God intended us to in a perfect loving relationship with a partner who returns everything we give into the relationship and more.
My equally long two cents.