My dog is a 3 yr old boxer his tail is hurting him he won't sit down on it propery he yelps in the light you can see 4 lumps on his spine what is wrong with him??

There's really no way for us to give you an answer, only a vet can.

He could have broken his tail, he could have a sore, he could have a growth, something could be imbedded in his tail... etc, etc.

The vet is your only option.

Or... he may have been stung by a bee or bitten by ants.

Either way, off to the vet you go