Are you sure you're not just seeing pinkness from the underlying tissue, much the same way that hatchlings appear to have pink "cheeks," which is not actual coloration, but the color of the tissue beneath the scales?
How was his last shed? Does he have any skin stuck on his cloaca?
What substrate are you using? Have you been away on vacation or in any way letting your cleaning routine slip? Are you disenfecting the tank regularly and keeping it dry?
I'm only asking the above to try to help. Overly moist and/or overly dirty substrates are going to be the most likely cause of a red, irritated underside on a reptile. Just trying to eliminate the obvious first...