Turtles can survive freezing tempertures, but actually do better inside during winter months. You need to get a UVB and heat lamp on him. warm the water gradually and even give him a nice long soak in 1/2 warm water and 1/2 Plain Pedialyte which will help with dehydration. During freezing temps they usually will dig into the bottom of the pond and stay in the substrate until it warms up. At this point continue to do the warm up slowly. He may not eat for a few days as he starts to recover from this episode.
I can locate a Herp Vet for you with a State or Country to have him checked out if need be. It sounds like you have done everything correct so far. Now he needs the light to bask and the UVB for the artificial sunlight as well as a cuttle bone for calcium. Joan
The pedialyte is sold where they sell baby supplies. Most grocery stores and Walmart has it. It is an infant electrolyte replacer. This will help replenish the electrolytes within the body. It will be absorbed through the Vent area. Joan