In a guys mind (and a woman's mind as well) a divorce is a major life crisis.
Basically, this man probably did all the right things: dating, committing, having children, being a good father, etc.
Divorced still happened. He followed his instincts, trusted his intuition and still, this path took him to separate from the woman he promised to spend the rest of his life with.
After such an experience, there is a strange after taste remaining which says something like: "what did I do wrong?"
The question he asks himself is: "what would stop this from happening again?"
Many second or third marriages do end in divorce. This is a fact.
I believe he loves you. What might be missing now is the trust for marriage. He simply doubts whether it is worth the risk.
What to do?
When you marry, you focus on the long term... You want security, right?
You can either wait for him to regain full trust (which might never fully happen) or simply focus 100% on the here and now.
This does not give you long term security. It gives you the power to enjoy every second you spend with him.
This is definitely the option I would go for.
I know it is a deep need in you. It is this desire to committ yourself for a lifetime together.
Now you can sign in for a life time every single minute you spend together. This builds immense complicity between the two of you.
There is something else: The "spirit" of his ex might still be very present in his mind. This means that he is not yet fully free emotionally...
Can you break through these layers of resistance? I believe you can but you need this extra power kick.
He needs as well to find answers and the assurance that history won't repeat itself the day he gets married with you.