@Jammie12 forgive me for asking but you mention he has checked out from being a Father, has he checked out from being a husband too? Is he still loving, attentive except where the kids are involved, do you ever go out as a couple? Does he EVER treat you in a way that makes you feel special, be it a hug, unexpected bunch of daffs or a flattering comment or has that gone too?
I asked because being blunt, to me there are two options, he has emotionally checked out from the relationship with you and the kids OR he has some mental health issues.
If he has checked out you deserve to know and to think about the way forward. Get a babysitter one weekend night and have a sit down chat with him, not confrontational, just honest. You both need to ask is it rescuable, do you want to continue and on what basis? Sometimes partners don't realise how significantly their change in attitude is affecting their family and need a wake up call. It may not be the outcome you want but at least you have the truth.
Mental Health - he could be depressed and needs treatment or he could have a long standing mental health issue that he managed to mask pre children (I am thinking autism etc or demand avoidance). That doesn't absolve him of his family responsibilities and the burden this places on you but it means he can't cope, not that he doesn't want to.
My brother was diagnosed at 55 and since then a lot of the issues in his life have become clear and had we known as it happened, we would have handled them in a vastly different way. He couldn't cope with changes in routine, noise or what he felt was unwarranted emotion - all challenges that come hand in hand with being a parent. So I am not saying your husband is autistic, just giving an example of what it may be and when it comes to mental health, if both parties what to get help, then there is some hope. People with these types of condition, don't want to come across like this but they are built emotionally differently and so the answer comes in a different form, one that does not come naturally to them. It doesn't mean they don't want that loving connection.
I think you need to examine for yourself, what has gone on in the lead up to the change in his behaviour and have a discussion with him to try and get to the bottom of the issue. Don't assume and be willing to listen without interruption. Does he want to be there and be a better version of himself or has he irretrievably checked out.
Once you know what you are looking at, you can decide your priorities and the way forward.