Hi Op
I can see you want for this to work, so ill avoid telling you to leave him.
I will say i have been in your DPs position. I was proposed to, said yes, then backed out and left after a couple of days of back and forth. We had started wedding shopping, i tried giving the ring back, we had to unwind our living situation etc... ExDP was devestated at the time. It was rough.
I can understand how he feels. He made, in his mind, a HUGE commitment and clearly took it very seriously, which is great. But being engaged can really smack you in the face and doubts can appear that you didnt have before. My DP wanted to move fast and have kids. This, deep in my soul, wasnt for me at the time and i just had to be honest. For him, it was a deal breaker (He quickly got with someone else, got married, and had kids - it was what he wanted). I am still not married or with kids, 11 years on. And its no regrets from me.
You need to be really honest with yourself and hand the ring back to him. Allow him to repropose when he feels ready - after youre finished with counselling. If you want to and can. He may never want to go through with it, can you live with that?
Kids, is surely, a deal breaker?
This is a tough one OP, i feel for you 