I suspect this a tough one to answer without being able to feel it. I don’t know much about greyhound physiology, but I know a very little bit about lymph node pathology. Firstly, are you certain that it’s a lymph node - could it be a cyst? Could it be explained by an insect bite? Secondly, even if it is a lymph node there are lots of reasons why it could be enlarged, infection being the biggest culprit. Something like sore teeth or swollen gums could result in enlarged lymph nodes in the throat. Does he have any other lumps in his throat/armpits/groins?
So, try not to worry too much - I