the substrate youre using should be good at holding humdity. it may be the size of the glass tank that is your problem.

are you using a screen top? did you try wrapping it with saran wrap(up to the heat lamp) or something to help keep humidity in the tank? do you mist daily?

get that humidity up and maybe he'll stop soaking. if he doesnt stop and he shows no signs of health problems then just let him soak.