Oh my, that's either a very nasty burn, a fungal infection, or both. I'd be getting him to an experienced reptile vet ASAP and I'd request that they do a "Skin Scraping Test" and send it off for testing, this is the ONLY way to diagnose a fungal infection, what type of fungal infection it is, and what Anti-Fungal medication will work to treat it. A lot of vets, specifically "exotics" vets or general vets, will not do Skin Scraping test unless you demand that they do it, they'll just look at it and guess what it is, and even end up prescribing a freaking Antibiotic "just in case it's a bacterial infection", and often times the Antibiotic actually ends up making the infection much, much worse because it's not a bacterial infection at all but rather a fungal infection, and only Antifungal meds will treat it, antibiotics given to treat a fungal infection often end in a horrible mess that lasts for months and months of the dragon getting worse, multiple vet visits, all kinds of tests, etc. So don't allow the vet to prescribe ANYTHING until they do a Skin Scraping test and send it out for the results, which usually take a few days. They aren't expensive tests at all and are very necessary...