It's not just his life, like you said it's yours too, because everything he does impacts you tremendously, sometimes family members suffer as much or more than we do during mania.
His sleep habits are atrocious, and that will guarantee him instability. BPs need a very regular daily routine and sleep schedule. Varying from that can cause lots of trouble with mood swings (mostly causing mania) and of course, more insomnia.
I am glad he's going to continue the Seroquel.
Maybe he should be changed to another mood stabilizer - lithium is a hard pill to swallow (not literally!) for many BPs. It's the least tolerated of all of them, but it does a good job if it's at the right dose. Mainly, the fatigue and muscle weakness are what cause BPs to stop it.
If that's a problem for him, he should also talk to his pdoc about trying something else, perhaps Depakote? Other than weight gain, it is very well tolerated. Of course there are the other epilepsy meds that are used, they may or may not work as well, but if it improves his quality of life and does work, it's worth a try.
You are going to HAVE to have a relationship of trust. You have to be able to talk, otherwise, you have no say in his behaviour. You will be the one to notice the signs and symptoms, often we don't. Or we just defend it. Have a serious talk about it, and if this is the way it's going to be, tell him you can't continue. He's sick, but also is responsible for his recovery, and if he isn't willing to work with you and his doc for EVERYTHING, not just what he picks and chooses to mention, your relationship is not going to work. Family members (especially spouses or SOs) have to be firm with us and not let us walk all over them, because we will, we know how to. He's a grown man; he doesn't need to be babied. He needs support, yes, but support is when he's trying hard, not when he is being defensive of bad behaviour and refusing to listen to YOUR feelings. You are half of this.
I was just like him the whole 10 years of our marriage, defensive, unwilling to get treated, yada yada yada, until he finally gave me an ultimatum. I guess that's the real test. It worked for me and I'm forever grateful to my hubby for putting up with me for so long, believing in me and finally demanding I get a grip. I've never been happier.
If he tries to call your bluff (very likely), stick to your guns. If you don't, it's an empty threat, and from then on he will know you don't mean it, and that you will still be there to take whatever. That is very important.
Sounds like he is making a small attempt, so there is hope!