Time, that's about it. You can do the water taming an all the other tricks. But with an old snake that wasn't taken care of, just take your time. You would be able to tame a wild snake faster then this one. Tap train him an handle for short periods.
Now, did the Vet check his eyes? Old snakes sometimes go blind, mine at 13-15y. Poor feeding can bring it on sooner.
Good luck!