Absolutely NOT. He's not even offering to make up what you will lose!!
And bullshit to 'help with childcare'. You cannot rely on him to provide childcare. And think about where you'll be if he decides he 'just can't' and you're faced with trying to find new, paid childcare.
Listen I'm not telling you NOT to move him in. Well, OK, I am telling you not to. But the bottom line is that you should change NOTHING about your life arrangements (childcare, transportation, work) and allow NOTHING to be taken away from your income or added to your expenses that he does not 'replenish' £ for £.
IF you decide to let him move in, you need to start off with him paying at least half of the household expenses, and that includes food, utilities, repairs etc. ALL of them. You can always 'step down' if you end up feeling he is 'overpaying'.
The issue of him paying part of the mortgage is, I understand, a tricky thing. Some have said it has no legal 'meaning', others have said it can give him a 'foot in the door'. Best you find out how it is where you live. And if you are advised by a competent authority (not MN or canvassing your friends) that paying part of the mortgage does not entitle him to 'home/tenancy rights' then he needs to pay part of that, too.
My feeling is to make it as financially 'unattractive' as possible to help weed out the cocklodgers. Cocklodgers aren't always poor or financially 'needy'. Some of them just want something for next to nothing to save their own pennies and pounds for themselves. And someone who makes 500 a week and only wants to pay 200 a month has the makings of one helluva cocklodger!!