I'm sorry your kitten's tail is in this state. From your description, it sounds like he either pulled all the fur off his tail due to discomfort, external parasites or stress, or he was in a cat fight and this was the result. If he spends time outdoors with his brothers, he could have been in a cat fight or he could have tried to get through a space that was too narrow for his tail and something scraped off the fur.
I'd definitely recommend an in person evaluation by a vet, so an antibiotic can be prescribed, to avoid infection, and for now, if it's bleeding slightly towards the top, you can gently clean there and the entire tail with Betadine scrub (diluted with water to the color of light tea), then pat dry and where it's bleeding, apply a thin film of plain Neosporin (NOT with 'pain relief'); it's not harmful if he licks it.
I hope all will be well with your kitten and please let me know how he's doing and what the vet tells you. Thanks!
Please be so kind as to rate my answer with positive feedback, as that is the only way I receive recognition from the website for my assistance. Thank you very much!
Providing a positive rating will not end our conversation, should you need me for any follow-up. Simply click 'Reply to Expert' for clarification or additional information, if needed.