Wow. Um. This is mind-bogglingly selfish of your husband. Hugely selfish!
Okay, now that I've gotten that out, I can kind of empathize with you. My husband has some mental health issues, and he's stressed out no matter what, it seems. While he clings desperately to me and our 3 kids, he acts like he just can't handle living with us. If a kid spills something, he starts hyperventilating and yelling at the kid. The other night, he decided our kitchen floor was "disgusting" and used our FloorMate to "mop" it. Two days later, he was super upset that there were crumbs on the floor after dinner. I didn't tell him that the baby had spilled milk on the floor and I'd wiped it up, or that we'd vacuumed up crumbs twice since he mopped.
He also literally starts having panic attacks when things aren't in their place. If you can imagine a house with 3 boys ages 7 to 1, a lot of things are out of their places a lot of the time. DH lets this prevent him from helping get the kids dressed, teeth brushed, baths given, pajamas on, stories read, and put to bed. He does none of it because things aren't perfect in the bathroom and the boys' bedrooms.
I think this sounds a lot like your husband because, I guess, my husband is a fair-weather father. As long as things are how he pictured them in his head, he's happy. But if something isn't perfect, he stresses out WAY more than is normal, and runs away from reality.
Another thing that reminds me of my husband is that my husband doesn't really take the time to research the special problems our sons face (our oldest just skipped a grade, and the other two are quite likely gifted, too) and won't listen to me when I try to tell him about the research I've done. He tries to handle the kids like most ignorant people would. For example, I researched the effects of grade-skipping on gifted children for weeks. I sent him one article (the best one that summarized all the information I'd found in those weeks). When I tried to talk to him about it, he spewed forth all the widely-held societal myths that the article I'd sent him de-bunked. But he didn't even open that article. He was dead-set against what my research and my heart told me was the best route for our son (much like you believe your way of handling your son's fits is the best way). Luckily for my son, DH's favorite sportscaster had skipped the same grade as a kid. DH emailed him to ask if it was a good idea to skip our son, and the sportscaster said yes, it probably was. I was so mad! Well, grateful that he was on-board with the grade change, but come on! He wouldn't read my one article (I tried to make it so easy on him!) but he gets his advice on raising our son from a SPORTSCASTER?
The fact that he is calm means that he's made up his mind, and he thinks it is the perfect solution....for HIM. He can avoid the unpleasant aspects of real life by dumping even more of them on you, but can stay married to you and keep the happy times with his son and your daughter. ----- The first time I confronted my husband about getting help for his disorder, he tried to keep me from leaving him by filing for legal separation. I know, it's counterintuitive (or crazy, one of the two!) The legal separation let him file a restraining order against me, so I couldn't leave the state with our children. He knew I wouldn't leave without the kids, so he effectively kept me from leaving. I had to go talk to a lawyer, who told me what I just told you. He said that my husband was only looking out for himself. He wanted to stay married for all the perks of marriage, but eliminate all the bothersome stuff, like working on his problems. Hot dinner in the evening, sex, and family time, without any of the responsibility. Sounds great, right? FOR HIM! It would have totally screwed me over.
And that reminds me of what your husband is proposing.
Frankly, I wish my husband would be up for a solution like this. Some couples where one has Obsessive Compulsive Personality Disorder (like my husband has) actually buy neighboring houses. The wife and kids live in one house, the husband in the other. The husband can keep his house exactly how he likes it, and the wife and kids can live like normal people (with the flexibility that children require). Honestly, I think that is the only way my marriage could last.
But your husband doesn't have a disorder, at least not that I'm aware of. And you said that your relationship is great, so there's no advantage to you if he were to move out.
I don't know why he's running away from his problems. Maybe he just needs a break?
Because I have been unable to understand my husband's behavior for years, I absolutely depend on a counselor to help me sift through the strange stuff and find the truth. I suggest that before you do anything, you speak with a counselor. But try coming up with a plan. Plans help me feel better about my life. Write down what you want to say to your husband, then run it by your counselor. He/she can help you decide what is reasonable and what is not, what is just you crying out in your pain, and what is actually going to help your husband (hopefully) come to his senses.
Coming from where I am, which is not where most people are coming from, I would tell him:
(1) I love you very much, and I don't want you to move out. I think it would be bad for our relationship.
(2) When you signed up to be a parent, there were no escape clauses. A father is forever. I'm trying to understand how you're feeling about this, and I want you to be happy, but I don't want you to run away from this problem.
(3) I am not willing to uproot our children, but if you would like to move out for a little while, we can try it that way. (Then you'll have to work out the details, like if he'll come home after work and eat dinner with you every night, stay until bedtime, etc. Listen to what he wants, but I wouldn't let him have his way with everything. Stand up for your family while trying to help your husband, but don't sacrifice the family or yourself for him).
I am very impatient when it comes to this stuff. I want to tell DH what I'm thinking and how I'm feeling right away, and tell him he's stupid for saying what he says and feeling how he feels. But I have learned over and over again that it pays to run it by your counselor when someone just isn't making sense.
Good luck to you, and let us know what happens.