working tax credits and partner may quit work/be fired

hi guys I'll try keep this short as I can, I'm right now on maternity and in middle of buying a house, me and my partner both working for same company although I have a higher role. I agreed with my boss I would return to the shift pattern as I was on before I went on maternity which is basically I can work anytime as long as my partner could get set shifts to work round child care. now my boss is saying he cannot set shifts and he will have to work what he is given, which is some kind of joke as I have seen him do this to other people and only give them 6 hours a week.

I went on tax credits calculator and found out my partner need to basically get 16 hours a week to break even with the tax credits that was based on my income alone. as the mortgage is going in my name solely I need to work full time and will only get the hours if I keep my availability anytime. my partner is very stressed at work as the boss just keeps threating him asking him if he want's to keep his job which I find highly unprofessional.

basically I think one of two things is going to happen either my partners going to quit or he is going to snap and be fired as he is trying his hardest to provide for us but he finds his job very demining and to be frank hates the way he is always spoken down to.

now I was wondering if one of the two scenarios happen what would happen to my tax credits would they then base them on just on my wage? would they go up straight away if it happens or would they wait till next year. I need to know as I have already started saving just incase this does happen. but still would like to know obviously my partner would start looking for another job he hate the fact already I'm going to be the bread winner.

sorry for essay just need to know
thanks