It sounds to me like he's groomed you for this. I would bet good money on him dropping your friendship like a ****ty rag if you turned down his offer.
This is what I immediately thought.

You could suggest looking at the topic again in another 5-7 years. That will allow you time to enjoy being a young adult before becoming a mother. It will also show if he is likely to be someone that will stick around. At 19 my DP at the time wanted marriage and babies, I suggested waiting til I was 21 - well he didn't stick around til then and I am forever grateful for suggesting this.

I personally would be steering clear of this man completely as something just doesn't sit right about the situation.