My husband is like this he does love you he believes you love him the same so he doesn’t feel the need to keep reinforcing it. Some men don’t know how to express their feelings.

It could be as someone else said, too easy to stay and too hard to leave. It could also be work is tough, or maybe he has mental health problems that just mask as being uninterested. I say this as I sometimes would probably appear this way to my hubby. I’m lucky he picks me up and makes me be involved in my family. Whatever that may sound like, I’m lucky to have him and he does it effortlessly. He could complain moan and whinge that I’m being a disinterested person but instead he plans fun activities for us and insists I come along if I had otherwise thought to sit it out.
I’m hoping I’m not criticised, I’m Sharing purely for the benefit of the OP to get some insight to how things are for a person, and how they can seem to the other half.
All the best x

I'm a little unsure what you're asking... What have you talked to him about? What do you want more from him? Do you want more help/ connectedness or time? What is it that is draining you?

Thinking about cheating on you at the first place is very wrong, until you communicate.

Since he's asking to marry, 101% means he wants to marry.
His actions doesn't means his words means, he is in some trouble.
Communication is the only thing that can clear things.
No one else than him can tell the real reason.
And also emotionally unavailable is also because of some real world problem, job, a colleague, someone from family, suppressed desire, orgasming too much also causes emotional detachment for guys, ask whether he watches porn too much, he will need to slow down there to get himself back and love for you.
Talk to him. Listen. Don't talk. Just listen what he has to say.
He is having some trouble because he's detached, not you. Become his support. Need of hour. He'll tell you because he wants to marry you. You are the lady of his life. Have a good time supporting your better half.

I’m saying this in the most polite way I can, but from my perspective maybe he might be too comfortable with you and doesn’t want to go through all the drama of leaving? Or maybe he’s thinking about your children reason why he can’t leave? Or maybe your over thinking things and he’s just exhausted from work??

Think positive and maybe bring some excitement to your relationship again.

Goodluck 🙂