C: tail injury

[Donna Anastasi]

One of my 9 week old pups has a swollen tail.  About a week ago, he got
nipped in a failed attempt to reunite him with his dad.  I thought it
had healed on its own but today I found a ball shaped swelling near the
base of the tail (about once again the width of the tail).

Is this serious?
Should I do anything besides putting on hydrogen peroxide?

Thanks for any help on this,
Donna