You could always try tweaking your temps a bit, just in case he prefers the basking spot cooler now that he is older.... maybe about 103 in basking? :roll: Your coolside temps could be raised just a tad as well... between 78-82 degrees.
At his age it could just be a hormonal/puberty phase he is going through.
Are you using the Arcadia 12% or a Reptisun 10?
If he is not losing weight and looks otherwise healthy... it could just be a phase and he will snap out of it soon.