Hello, I'm Dr. Deb.
I recently came online and see that your question about your boy hasn't been answered. I'm so sorry that you've had to wait for a response,but if you still need assistance, I'd like to help if I can.
There are several possible explanations for why your altered dog would be attractive to other, unneutered male dogs:
a) If, for example, he was a shelter or rescue and had a retained testicle, then this testicle could be producing estrogen which makes him similar to a female dog in heat.
If this is a possibility, then estrogen levels can be run and he'll need surgery to remove this testicle.
b) Exposure to estrogen either through any drugs he may been given or exposure to any human hormone replacement therapy can also cause this behavior from other dogs towards him.
2. Urinary tract infections can cause this behavior. If suspected, then a urinalysis should be helpful in determining whether or not there's an infection present; appropriate antibiotics should resolve the problem.
3. Infections in the prepuce (which houses the penis) can also cause some dogs to be attractive to other dogs although it sounds as if the unwanted attention is somewhat intense for an infection in this area.
Close examination of the prepuce may reveal a problem.
4. Anal gland infections can also cause this problem but I'd expect such an issue to be fairly obvious when you looked under the tail.
I hope this helps although, again, I'm sorry for teh delayed response to you. Deb