Help noticed lump

http://www.peteducation.com/article.cfm?c=2+2111&aid=424
I can't see ypur picture but it can be almost anything. If the poor guy is picking at it, I don't think you should just ignore it. Could be flea allergies, bee sting, could also be worse. At the very least I imagine you wouldn't want him to chew/itch his tail raw. You should get a professional opinion methinks, then again I'm a worrier.

If its something that doesn't belong there, and if its bothering him, you have two really good reasons to have him checked out by a Vet.

how close to the base of the tail (can't get much orientation in that photo)... if on top of the tail near the base could be associated with the gland most dogs have in that area. But agree with other answer... a bump can be just about anything... takes a biopsy to know for sure what it is. I would have it removed and checked out.