My dad had something similar after he smashed and I do mean smashed his wrist. He broke it so badly that all of his wrist bones were floating in his arm. They used pins to spread his bones apart hoping some of his bones would migrate back in place. He has had a numb trigger finger ever since this accident. He can use the finger just fine but has no sensation in it so when he uses things like a saw his finger tends to release on the trigger so to speak without him noticing it and the saw will quit running. To the best of my knowledge there is nothing that can be done for him as they told you in your sons case. I would suggest he keep on with therapy but if insurance decides to stop paying for it then substitute a home therapy routine. One other thing that might be of benefit to him to is to join a fitness center to work on that extremity. My father in law had a nasty plane crash and broke his leg in several places as well as other bones and once he was officially done with therapy he worked out on his own at a fitness center which helped him tremendously. Best of Luck to your son in his recovery.