Obviously you know your partner better than anybody op , but so many alarm bells are ringing here.
The most obvious one is the small cm payments he makes , are you sure he's not pulling the wool over his ex partners eyes? Or worse , and he's a bit of a nasty character , and she has had to accept the the small payment as he messes her around if she speaks up. I know about these situations , I've been there , I'm living it.
Also , he's happy for you to use the entire lump sum towards the house , then leave you much shorter than him each month after mortgage payments. Then he makes you pay more still , because you're female and have a child , yet I don't see any mention of how much it costs him when his child stays with you.
I'll be honest with you @happydays67 , he sounds like a nasty penny pincher, and ridiculously selfish. He gets to live in a big house , pay a tiny mortgage , his dc paid for when they stay , he doesn't provide for his child much in general , reading between the lines he gets a lovely step mum too. He also potentially get 50% of the house if you marry then split dependent on how tightly ringfenced your lump sum is.
You however are getting stung , and don't seem to be enjoying the privilege of your inheritance at all. This money is to make your life (with your child) easier , but your partner is reaping far more benefits here and it sounds so very wrong.