correct me if im wrong but from this post it seems like you have a hot side and/or an enclosure ambient temperature that will get to 90 degrees at times and a UTH under the cool side hiding spot. im doubting that the UTH is regulated by a thermostat or even a rheostat and i dont know if you are measuring the temp of the UTH with a digital thermometer meaning the UTH can be extremely hot at this very moment.
if this is the case then the snake has no cool side and may very well be soaking because of the high temperatures on each side.
i may be wrong but it would be a good idea to double check your ambient and basking temperatures and try to create a proper warm and cool side. cool should have no UTH with an ambient of 80-82 and warm should be anywhere from 88-92.
if your humidity is good and he has no mites then he may just be soaking for fun... if not then it may be too hot in there for him so just double check.