Try and put some more sand of different textures and sizes around his burrow along with some very small rubble/shells etc.
He should take all these things and make his home just as he wants it.
It's normal for him to dig a hole and post up in there, thats the fun thing about Jawfish, their like little troops in their bunker/foxholes.
With Jawfish, they build their own home, you just need to provide them with their "building supplies"