The middle always seems most balanced and safest for me. Think of it this way; if you were to marry, you might be married 50 years so this, being the "beginning" is just part of the up and down of the path. I would sort of combine 1 and 2:

Stand by him and help him unpack this bit of emotional baggage for a short time by going with the flow and accepting, while encouraging/making it clear to him that, together, the two of you can move through this.

I don't think you have seen this before in him so I'd give him the benefit of the doubt and see what he makes of it; if it becomes a negative pattern and/or you can't figure out how to help him better, then you let go and move on, but leaving him a better man for having known you.

I am my husband's second wife and when I think of what he was probably like when she met/married him and what the man I love is like today; I'm very grateful to Time, Life, and his first wife If you cannot stay, leave him better off and a better person for his next love?