Can your DH make some blocks to add to the back where the missing fabric should be?
Can you add a contrasting piece of fabric?
Can you get some more of the fabric that you are going to bind the quilt with and add that?
Can you make some scrappy blocks to fill the gap?
This is an accident but I do appreciate how much more upsetting it is when it is your DH first quilt. Sorry for your distress.