One of the terms of my divorce decree was for ex-spouse to assume home mortgage which is currently under my name. Additionally my decree stated that ex-spouse shall execute any and all documents and perform all acts necessary to remove me from the mortgage related to the real property. It's almost been a year and he has not assumed loan. He is telling me as long as he makes payments on time he does not have to assume that loan. Can I sue him for violation of divorce decree in the state of Texas? Also, at the point will judge tell him to refinance loan under his name?

Yes you may take him back to court for contempt.
Just making the payments is not assuming it.

Now of course if he can not get a new loan, if he proves he has tried and is turned now, it may not be any other way.