
First of all try not to panic. However, I have to say I felt a significant mass in the neck of my old boy, years ago now, and had my vet check it out. She didn't like it at all, and took a fine needle aspiration from the rear lymphs. Sadly that came back - lymphoma. I have to say, given his age at the time, we opted for no treatment and in all other respects he was in rude health. And that continued for 6 months at which point I was doubting the diganosis. Every time he twitched, I thought that was it, but in the end, he just shut down, didn't come for his supper with the others. We took him to our vet who gave him a shot in the hope that would get him going again. Sadly not to be and the following morning we ended it for him. BUT he had 6 months of first-rate quality of life and he was in his 13th year at the end.
I am feeling another mass in the neck of my current hound, but then he's riddled with cysts for some reason (some are in my breed and I've never discovered why some and not others) and there's no way I'm having any more removed - we had one at the set-on of his tail taken off and another on his shoulder, but no more unless they grow.
Hopefully this will just be similar with your boy. Cyst.