What would you do?

My ex (divorced 6 yrs ago) has never taken my kids for his summer mth (rarely comes on his wknds) well this yr he found out I am pregnant with my new husbands baby and due during "his mth" so wants to take the kids now. I dont have an address for him, he denies me contact when they are with him, mentally abusive, and the kids just informed me he is on meds and I dont know what for but was told by his family member he is very depressed (that scares me BIG time) SO....would you allow him to take them for the mth? Or ever again for that matter? HELP! The kids refuse to go back cause of the way he treats them.