I agree; they should look the same on both sides. I'm not sure what's going on there, it could be an issue like the original poster had, or it could be something simple like a retained testicle. Sometimes the testicles don't descend out of the body cavity into the scrotal sacs, which could make one look "deflated." It could also be torsion or some other issue--not necessarily cancer--but definitely get it checked out soon. If you were planning on having him neutered, this may be the perfect time. If not, he may not be a good quality breeding buck as it is.
Mom to Tony, Muffin, Frida, and Big Bunny