What are the possibilities of getting a FSTE approved to delete my husband's orders and getting a common DEROS?

There's really not enough information to give a good answer here. Technically you can't have an FSTE if you're on assignment. A lot of variables really. How long have been enrolled? When are your DEROS dates? Where are you? What assignment is he on? How long have you both been at your location?

Less likely his assignment is deleted and more likely that you will simply receive a follow on assignment to his location.