OP, correct me if I'm wrong, but from your posts, your DH earns £20k a year for 60-70 hours work a week, and has been working in the family business for 15 years?
If that's correct, his hourly salary is £5.92 for the average of 65 hours a week. National Miniumu Wage is £6.08 if you're over the age of 21, and if he works in London, the London Living Wage is £8.30
Can any posters on here tell me that for those long hours, and for the number of years he's been at the business, they think that's fair remuneration?
OP, it sounds like your DH isn't actually bothered with what he earns because it "family" and because one day, long long from now, he's get "something" from the business.
Perhaps if you put is wages into perspective (i.e. his parents are actually paying him less than is legally required in this country for the hours he works) then he might be more inclined to speak with them?
If not, I agree that a quiet lunch with MIL is in order. At this lunch, you can tell her that, with a heavy heart, you're going to have to put your LO in childcare and go back to work full time, and that your DH may need to give up working as his salary, being under NMW, isn't enough to cover childcare.
I also agree that it's time for a budget sheet for you to show your DH. You can also tell him that you think it's admiral that he works such long hours, but explain to him that unless he's actually paid hourly, then all he's doing is devaluing himself, as his salary doesn't change if he puts in all that extra.
Perhaps all that's needed is for your DH to take a stand and say "I won't work more than 37.5 hours a week for that wage, and if you want more out of me, then pay more".