I agree with everyone else on here, even though I'm sure that's not what you wanted to hear. If it means the dog's health and happiness, there is no reason to not have him neutered so he isn't suffering. He's not only suffering with this erection problem, but also from the frustrations of not being able to mate and with males that is a 24-7 problem. By keeping him intact for this long you are putting him at risk for testicular and prostate cancer and just forcing him to go through agonizing erections that won't go away. There is a lot to know about breeding besides putting two dogs together and getting a litter of puppies. Like dragondawg said, you need to research your dog's cancer history, his genetic history, his pedigree. He needs to be tested for any genetic problems he could pass on to his offspring. And that doesn't mean a check up by the vet. Do your dog and any future dogs a big favor and have him neutered. You'll only be contributing to the homeless dog overpopulation by breeding him. If you want another Boxer and you know people that want a Boxer (since that's probably one reason you want to breed) adopt one from a shelter or rescue instead of bringing more into this world that could end up homeless.