Congratulations on your baby op!
I totally agree with this post ^^. The initial arrangement between your dh and your mil was intended to be mutually beneficial. She had him living nearby and he got on the property ladder. Once one side of that agreement is withdrawn, then I think it is up to the other to compensate.
However, I don’t understand why your dh thinks it’s necessary to pay back more than he was lent? Is it a lot more than just the interest? Unless your mil really needs the money to survive? Maybe she isn’t as well off as you assume? And she needed your dh’s financial help as much as he needed hers?
Does your dh have siblings? If not, presumably your dh will inherit this asset anyway in the long term?
I can understand you not wanting to live in the same house as your mil but don’t dismiss out of hand the financial benefit of having a live-in baby-sitter once your son gets older and starts nursery and school. That would enable you to earn and progress more in your career. Childcare costs thousands too remember!
I agree that your husband sounds like a really decent man and I wouldn’t tbh make renting out to be a massive problem if it’s only temporarily.
As pp said, you will be free from financial strings and obligations, and sorry if this sounds harsh, but it sounds like you and your dh didn’t wait long to have a baby before you were more financially independent.