What are the temperatures of the 3 temperature zones inside his tank ? (Basking Spot Surface Temp, Hot Side Ambient (air) Temp, and Cool Side Ambient (air) Temp) And what type of thermometer are you using to measure his 3 temperature zones? You must have either a Digital Probe Thermometer or an Infrared Temperature Gun to be able to measure the Basking Spot Temperature, it's a "surface" temperature, and if you're using any type of stick-on thermometer they cannot measure his basking spot surface temperature, and in addition, all stick-on thermometers are very, very inaccurate, when tested they are usually off by between 10-20 degrees, so if you don't have either a Temperature Gun or a $10 Digital Probe Thermometer, you need to get one ASAP and then allow the probe to sit on each spot you're measuring for at least 20-30 minutes before reading the temperatures.
Also, what
UVb light does he have? How far away in inches is the
UVb light from his Basking Spot/Platform? Is it a long, flurescent UVB tube, or is it a compact or coil/spiral
UVB bulb?
If his temperatures are too hot, or if the
UVB light you are using is inadequate (specifically if it's either a compact or coil
UVB bulb and not a long tube) then both of these situations will cause him to hide. Obviously if his temps are too hot then he's hiding to cool down, and most all coil/spiral and compact UVB bulbs cause serious eye problems, blindness, and neurological damage and will cause them to try to constantly hide from the light.