In the very short term, the advice above is great.
One other thing you need to do, is to start a log of your H's time spent with your children. Documentation. "Took S2 at X time. At FIL's until Y. Overnight at other location. Returned at Z time."
At the least, it will help you to have a reference so you can remember accurately, and know whether their contact is increasing or not. Worst case, you will need it to demonstrate that he has not had regular contact, and that there was inappropriate exposure of S2 to cohabitation.
