They do not understand the special bond and love a man and his wife have and how different it is than the bond and love a man has with his kids.
After seeing their father married and divorced so many times, I'm afraid they've come to understand that the love between a man and his wife is disposable, and father/child is the only constant.
You sound angry at his children. Are you? Does that play out in the household dynamics? How does he treat your son? Blended families are difficult by nature, and take strong boundaries and understanding to get them working in a positive way. Especially when there are teenage step-kids.
I'm going to assume that you and he get together when the kids are at mom's, and he takes advantage of your willingness to perform 'wifely duties', am I right? That may go on forever unless you put an end to it. Have you read up on Plan A and Plan B here? It's for infidelity, but I think following a modified version may work for you here. Using the carrot to lure him back, showing him what he'd be missing if he left for good (and there's still a large chance that may happen--he's got quite a record going there), and then moving into Plan B, where you cut off contact unless and until he decides to come together as husband and wife, full-time.
And Plan B will give you some time to really consider whether you want to continue this marriage or not. From what you've said, he packed up and left because of an accident? And has now been gone three months?

Give a look at Plan A and Plan B, and also the 180, and get working on something other than hanging in limbo.