HI, this sounds exactly like what one of our Cairn terriers had on his neck, like a lump of gristle beneath their skin but not attached to their bone or muscle? We took him to a vet, unfortunately I can't remember what it was called but it was completely harmless to him, even though it did get slightly bigger with time, we just took him in regularly to make sure it wasn't becoming an issue. So take him to the vets and don't worry too much until you get the full picture