Hi there, it is just a very slight injury, possibly from a bite when he was with other babies. Be sure he's kept alone and get some betadine to put on it twice a day with a Q-tip or small cotton ball. This is very common and not usually a problem at all. Just watch that it doesn't get worse, then you might need a vet visit, possibly antibiotics but I doubt it will get any worse. Once in a while the tip will fall off and still be fine as long as infection doesn't spread up the tail.