Hi jfmcrazy, I work in a spay/neuter clinic down in south jersey. I can tell you that it is a common complication for older/larger male dogs to get a scrotal hematoma. That's why I always recommend having a dog neutered before they have a chance to get too big
Please seek a second opinion. Feel free to PM me if you need any help since you are somewhat local.
Do you know what kind of meds he is on? Usually cephalexin works very good for post-op infections in spays/neuters. Also if your dog is in pain like that even my shelter would likely put him on some sort of anti-inflammatory (such as rimadyl). It's pretty insane that the vet is making you come back all these times, not giving you a direct answer and since this is a private practice he really should be medicating your dog a bit better.
Also, if you have any way of doing so, if you could post a picture of the surgical area I can give you just a general medical opinion, but of course I am not a vet, but have no problem comparing it to some other cases I have observed.
Hope your doggy feels better soon!