Looks like it may be a fungal infection.
Is it like that under the tail as well ? Find a good reptile vet to do a skin scraping to be tested for some kind of fungal infection.
Have you soaked him at all to see if it's just a nasty stain, is he housed with another dragon ? They will climb on each other + at times may poo right on their sibling. See it this comes off at all in a warm soak, buy some betadine at the pharmacy, add to the water until it looks like iced tea. Just be sure the baby doesn;t drink it. Then use a paper towel or toothbrush to see if anything comes off.