I'm not sure what you're referring to on his chin...I'm not seeing anything I'd be concerned about. If you're talking about the "dashed line" of darker scales on
either side of his head (underneath, first 2 photos) that appears to be normal coloring, maybe you never noticed before? If you mean the central "cleft" that's
normal too. The only thing I see is on top of his head, those rough looking scales, & I can't be positive from here, but I've seen something like that before, it's
not serious but something you want to treat topically before it spreads to other scales IF I'm correct- it's a fungal infection that BPs are prone to (since they live
in warm humid enclosures!) but to be sure, you should check w/ a vet- online isn't the proper way to diagnose things. You can wait a bit, to see if it's still there
after he sheds, and to see if you start seeing other single or little groups of similar rough-looking scales.