The heat light may just be drying out the air more, making him a little bit uncomfortable and unwilling to come out. In addition, there is anecdotal stories that perhaps reptiles can perceive/see red light, instead of it being invisible to them as what is commonly suggested, so he may just feel like it is too bright out for him to come out.

Try a ceramic heat emitter instead, and see if that doesn't fix the issue.

-Jen