Wow. That must be a lot to take in. Am so happy you got to hear that from him but I understand your reticence. My first thought is: Proceed with caution.
In my case my husband said he wanted to come back after 9 months but I could tell he wasn't vested in marriage counselling. I think it was a major touch and go as things were not going well with OW1 and he thought I could slip off the line. The rejection was even worse the second time.
One thing I have learned is that time can be our biggest ally. You need to process all of this. Plus making him sweat it out while you do is irrelevant. I think you are right to be wary of his ability to flip flop. Move at your own pace not his. He is starting to see what he could lose.
I am sure much wiser people will weigh-in and give you great advice.
Me-54 H-49 T-1. M-7 BD 6/13 ILYBN I threw him out OW - 3/13 OW2 on and off Overlap w/Ow1and OW3 OW3 - 8/17 H filed 1/17