He is absolutely right, this is a condition called Acanthosis nigricans. Here the thickness of the skin particularly around the folds like neck, digits, under eye slowly becomes dark . It is seen in obese people or overweight people,
thyroid disorder,
Diabetes etc. Most common cause is weight gain in the recent past. So please plan to gradually get back to normal weight.
Next Steps
Apply Grazia up cream at bedtime. Use a good sunscreens in the morning