The poor thing, is probably crying, because he is in pain! That is a super sensitive area, and if it red and swollen, there is something definitely wrong and there can be a number of things causing it. Please, for your dogs sake and health, do the right thing and take him to the vet as soon as you can{TODAY}! Please don't put off getting him to a vet today, because it needs to be diagnosed, treated and it's not fair, to let the poor dog suffer in pain,when it can be treated! The vet will be the only one, who is going to be able to tell you, exactly what is causing the problem, and take the proper steps to treat the problem. I hope he feels much better soon, and that it's not serious! Give him a big hug and kiss from me!