Does this dog do it to any other dogs? If not, I'd suggest you take yours to the vet, even if there's no sign of discharge, something is causing this.

 

I agree with the above. Get your dog checked, just in case. And do not permit that dog to make contact with yours. You can prevent your dog having to put up with that behavior, so protect your dog. Don't continue to let him be bothered by another dog doing something he hates; just walk away when you see that dog coming.

 

I agree with the two above statements! If walking away doesn't quite help and the dog gets uncomfortable seeing the offender then you could try switching walking times just a little. Assuming that the walls that this has happened on has been at the same time or about the same time. Because that just isn't ok!!