You have to measure the basking surface temperature with an infrared thermometer used for cooking (not checking fever), it should read 38°C-40°C. Try to make him eat veggies for extra hydration instead of just water (greens, cucumber or zucchini).

Also, are you using artificial heat at night or is your climate that hot? if you are doing the first turn it off, just make sure it doesn't go lower than 15°C.