Your husband should only file joint with you. Period!!! Actually his ex-wife should file separately as Head of Household if their are children, which would truly benefit her greatly. The rule is that whatever your status is on 12-31-2017 is what you file as. Your husband might be trying to achieve a benefit of claiming a lot of deductions that he probably does not have with you yet. Example mortgage interest, child credits, etc.
You need to have a heart to heart with your husband to find out what it is he is after, not trying to start trouble here. He just might be trying to come out ahead financially.
Remember though if you do file Married filing separately, which they would surely crosscheck to see that he is doing the same. This is the highest tax bracket there is. I know I wouldn't do it.
