Noticed a...boil looking thing on Nicodemus' tail. He is my most recent rescue. I don't know much about him, but he appears to be over 2 years old and very thin, despite lots of daily feedings of high calorie foods. I couldn't get a very good pic of it, but here's what I got...

It looks like a raised pimple, with a black center. Here's my big worry. At the base of his tail, which is a few centimeters below the boil, I think I can see the bone in the tip of his tail! It's like there's no flesh over it and when I put my fingernail on it, to see if it was skin or bone, he about jumped sky high. Could this be some
flesh eating thing?

Looking back on his pictures from the day he arrived, his "tail bone" was already showing. Should I be worried, or could this have been an old injury? Anyone have any experience with this? I'm worried it might need to be amputated. The end of his tail is not inflamed or anything. Think I'm going to set up an appointment on Thursday, to be safe. His weight is still 425grams and holding, but I keep thinking that something must be keeping his weight so low. I don't know how it could be an infection as I have him on Baytril and Trimethoprim Sulfa, already.