Right now it just looks like a partial leftover shed or irregular shed. Keep an eye on it, if it starts turning yellow then you should get him to a reptile specialist for a skin scrape to be analyzed.
I agree that it just looks like an area about to shed. Look for darker yellow or skin that looks wounded/scabby. At the moment it just looks like dulling skin about to shed though.
It's possibly a stain, but the even pattern and the dull color look like shed to me. You could give him a bit of a soak in some warm water to clean him up a bit.