I suppose the best place to start unravelling this would be to sit and talk honestly about it with him at a time when you aren't in bed and things are calm, trying not to allow your frustration on the subject to show through.
I honestly don't know why he's acting that way, perhaps he does just genuinely have a very low sex drive, or maybe there are other reasons for him not wanting sex- anything from bad past experiences, changing feelings for you sexually since you having the children, erectile problems, feeling embarassed about his body, depression, and the effect antidepressants can have, even to worries about his sexuality- I really am guessing at reasons here, it might not be any of things- he might just prefer a good book!
You can't expect an overnight change but if he hears how sad and worried for the future it makes you, and that it makes you feel unattractive or rejected, he should at least feel moved to give you a bit of honesty too. You need to keep talking, and make sure you have plenty of non sexual physical contact (holding hands, cuddling, kissing goodbye, hello etc- even playfighting) so you get used to touching each other again, almost as a warm up to the main event.
But you may have to accept that he may never come onto you, maybe he just doesn't like initiating sex, fulls top. Can you live with that, as long as you are having sex?
If talking fails, and you don't get anything from that book, there's always relate, if he'd go? and while it doesn't help with every aspect of the situation, maybe a toy of some description might help you feel less frustrated so that you can talk to him calmly without sexual frustration escalating the talk into an argument?
hth, it's a tough one to advise on.