I'm at a loss here. But I have two guesses. One is that his habitat lighting is too dim, or his habitat is too cool. Another guess might be that he sees himself in reflections off the inside of the glass or plexi. If the inside of the cage is dimmer than the outside, he can see his reflection, which would most likely be upsetting to him. If the inside of the hab is brighter than the outside, he can't see reflections on the inside of the glass.
Is it possible that the habitat is too small? does he have a hab end or corner where the temperatures are lower than his basking spot? Id say the cool end must be 80F for a baby, with places to completely hide.
When you take him for a relaxed visit in the full midday sun, does he turn dark or even black? Some igs do that when they are cold and want to soak up as much heat as they can from the natural sunlight.
After all these years I've read posts about similar things that you describe. I think your ig is healthy but could use some cheering up in full sunlight.