His tail is perfectly fine and is not part of the problem at all, it's dark because #1) some beardies have dark tail ends, and #2) their tails get even darker when they
black beard...he should never be put on Baytril without having a culture done to diagnose a bacterial infection, nor on metronidazole, which IS ANOTHER ANTIBIOTIC, IT'S FLAGYL, for yet another reason that is undiagnosed, plus Panacur? So 2 harsh antibiotics (Flagyl is a terribly strong antibiotic and can cause great sickness in them, it DOES NOT KICK-START THEIR APPETITE AT ALL, IT DOES JUST THE OPPOSITE) and then a dewormer...
What led you to take him to the vet in the first place? That's the question that needs answered. His problem right now is that he was given 3 days of Panacur (he does not have worms anymore, if he ever did), 2 weeks of Baytril, which is dangerous, and the Flagyl on top of it, and all 3 of these medications were given to him based on not 1 diagnostic test, not a fecal or a culture.
He is extremely sick from all the meds, and extremely stressed from all the meds and the vet visits. Your
UVB light is inadequate and absolutely does need replaced to a 10.0 T5 tube, either a Reptisun or an Arcadia, but he's not eating because of all the meds and the stress of it all. I don't know why you took him to the vet originally, but if I were you I would stop ALL meds he is taking now, keep feeding him the Oxbow and offering him water several times daily dripped on his nose, and let him recover and destress. And do not ever take him back to that vet ever again, they don't know what they're doing, it was extremely dangerous to give a dragon 3 days of Panacur then 2 weeks of Baytril along with Flagyl on top of it, he could have died just from the meds.
I assure you though, his issues have absolutely nothing to do with his tail, it's perfectly fine and looks nothing like "Tail Rot". Tail Rot is an infection that spreads up the tail, killing all the tissue along it's way, and the tail has open, necrotic tissue that literally shrivels and falls off before it spreads any further, it's not just darkening of the tail. I promise, forget about his tail and concentrate on trying to find a reputable, experienced reptile vet with bearded dragon experience for the future. Please, get him off all meds, and under an adequate UVB tube, check his temperature gradient to ensure it's correct, and keep offering fresh salad and live, gut loaded bugs every day in addition to giving him the Oxbow and keeping him
hydrated.