Separation and Taxes

My stbx and I filed an extension to file our taxes in which we owe aprox $3,200.00 by Oct 15th. In all prior years we have filed married/jointly. I think since we owed taxes that he has went ahead and filed married/separate leaving me with the tax burden to pay by Oct 15th. Can he do this? Or is he still responsible for half of the tax debt that is due?

The tax debt is martial in nature and can be distributed as part of Equitable Distribution.