Speak to a solicitor and find out the reality of your situations.
From what you say, he is basically slave labour.
Look into national minimum wage - and pension law. His parents may be breaking the law.
Find out the reality of willing and inheriting farms under today's conditions. What is his and your children's future realistically going to be like?
Bear in mind that as your husband he co-owns your house, its contents, your income, savings, pension.
Find out the likelihood of him being able to get 50/50. If this happens, your children will be living in that farmhouse with those people away from you for half their time, with you paying for it.
You'll need to gather ALL the evidence and the practicalities.
What's the distance from school like?
What hours do you work?
What's the current and historic childcare duty split like?
Do a detailed breakdown of exactly what you do and what he does.
How much you spend on absolutely everything child and home care related. Records and receipts.
I don't think he'll ever change. He's probably the latest product of generations living this way - is it an old farming family? Has it been in their hands for a long time?
He's been infantilised by them, his childhood bedroom is preserved so he'd go straight back there as the cheap live in farmhand. Generations of fear, obligation and guilt are just playing an old story out once more with him. All ambition and independence stunted out of him. It is in effect, cultural.
And he seems semi detached as a family man. You were together as teenagers; were you both inexperienced and did you have the children young?
Is there any indication your children might be sucked into this too? Are you in fact breeding and raising up their next generation of dirt cheap farm workers?
You need. Professional. Help.