Susie
We lost our boy to Lymphoma in May.
We first noticed a problem when his lymph nodes were up in his neck, visibly up, having said that they did not cause him pain, so hopefully your baby will be ok and it could just be an infection.
Even if he does have lymphoma and hopefully he hasn't then with Chemo, which I would recommend, in a young dog like yours there is a good chance of survival.
Have they taken a FNA of the lymph fluid and does he have any other lymphs raised? If its just the neck lymphs then from what our vet told us it is more likely to be an infection which can be cured by anti bionics.
I hope he gets better soon.