If your spouse is legally married, but does not have dependents living in his home that allow him to file as HOH, then he must file as married filing separately.
However, the laws on tax dependents treat stepchildren the same as biological children. So if it happened that one or more child lived in his home more than half the year, he can use that child to qualify as HOH and you can't, even though you are the biological parent and he is the stepparent. The key factor is where the children live the largest number of nights.
Also note that married filing jointly is still an option. While HOH is better for you than MFS, MFS may be worse for him. It is possible that your overall tax picture will still be better filing jointly, although you can only determine that for sure by testing it.
