It can be a case of extreme blue balls or testicular cancer.Either way it has to hurt really bad.You need to take him to the vet.If your strapped for cash there is one thing you can do to evaluate the problem.Their organs work just like peoples do so you may have to be more than a friend to mans best friend.If the semen is dark in color you have a serious problem.If it is regular in color give it a few days and he will be ok.don't let him breed a female until you or a vet has helped him.
