J.S.
I would be very upset, too. But if you are ABSOLUTELY SURE that his mom IS able to stay the night with the boys that he is supposed to be with them, you can look at it as HER time with her grandsons, and that's ok. If you still have a good relationship with her, it's ok to ask her how she's doing with the boys on those nights. If she's really not doing ok, then it's time to address it with your ex.
I think it would be a completely different story if he was hiring an overnight sitter on those nights, instead of calling his mom. It may also be that he's afraid he won't know what to do IF one of the boys got sick, and he's not about to tell YOU that!!! (Men can come up with the most bizarre "reasons" for doing some of the things they do!!)
At this point in your situation, there's not a whole lot you can do, except be thankful that he IS asking his mom to watch the boys on his night. If you proceed with the divorce, be sure and ask your attorney about stipulations for visitation, that would include your husband being required to stay at home on the nights the boys spend there. (The atty will tell you if you can request that stipulation from the judge, or not.)
I've been in your shoes once, and I'm so sorry you're having to wear them now. Feel free to call me or email me anytime, even if it's just to "vent" your frustrations!
Praying for you!
~J.~
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