I don't see any definite signs of Y.F. As the others mentioned it looks like natural color and/or skin shedding. If you are truly concerned, have a skin scraping done by an experienced ONLY vet. Some vets are called " exotics vets " but actually treat mostly small mammals, birds and a few reptiles. They can do more harm than good in many cases. I've seen several beardie owners take the dragon to a vet that prescribes antibiotics " just in case " because they have no clue but they feel the need to prescribe something. Antibiotics are useless for Y.F. treatment, so the owner wastes their money and puts harmful meds. in the dragon for nothing.
Anyway, I think he's fine but if you have a reptile specialist nearby then have a skin scraping done.