Tell him he can either save himself the extra £100 by agreeing to pay you the CMS amount now and in the future, or that you will go through CMS.
He has a high salary and should not be complaining about paying the minimum. Who cares if you live rent free right now. The CMS system is crap compared to some other countries and realistically (depending on which part of the country you live in) £500 is not going to cover food, clothing, activities for your child, school uniforms, childcare/wraparound care AND accommodation for your child.
Your circumstances have nothing to do with how the CMS system calculates maintenance, so if he wants to pay the minimum manadated by the CMS then he has to follow their rules which are that he pays according to his salary and that he pays their fees.
You have to be firm because he sounds like the type who will constantly be looking at your situation to justify paying less.
My ex-h earns nearly £100,000 but has seriously asked the judge here in the UK to pay £250/month in maintenance (he lives abroad so the CMS doesn’t have jusridiction in our case). He currently pays £1,000 decided on by the foreign court.
When we first split I didn’t insist on a reasonable amount of maintenance and he avoided paying anything for as long as possible, then when it was court ordered he stopped paying and I had to go back to court to get it taken out of his wages, then he started fraudulently claiming child benefit for our child abroad by forging my signature and I had to take him to criminal court.
I have had moments when I’ve had some sympathy and thought well yeah it must suck having to pay child maintenance, but then I remember that I am responsible for paying for EVERYTHING for my child and children are expensive. Keeping a roof over her head is my responsibility regardless of my income, if I lose my job etc. so it’s fair for the other parent to contribute a reasonable amount