If you didn't know that your turtle overheated then the first thing you would do is get an X-ray. It is very strange that the vet didn't take one. If it was my turtle I would still get an X-ray (see highlighted text below). The overheating on the dock could merely be a symptom due to his behaviour change and not the cause of the posterior leg paralysis as you believe. There is also the possibility that your turtle overheated because he hauled out and refused to move from his dock due to one of the 5 major causes mentioned above. I wouldn't dry-dock your turtle but return your turtle to his aquarium, making certain that he is able to swim to the surface using his front legs. If not, you will need to set up a shallow tub with a small heater and enough water to cover his shell but shallow enough so that he can breathe when he stretches his neck out.