Can't help but think that his main motivation is financial. However, it's worth keeping in mind that teenage boys often want to live with their father anyway - it's an important father-son bonding time. The best you can do is keep a clear path home. I would fight for some visitation though, just to enable you to keep in touch and make sure they are not suffering at their father's hands.
I've seen this happen to some other women here on the boards, usually it takes a while but eventually the kids find their way back to you. Your ex won't be able to keep up the facade with them forever.
