Hi, understand your anxiety. It could be a branchial cleft cyst, they are firm and sometime enlarge if the are filling with fluid. These have to be removed usually but are not serious. Sometimes infants are born with a condition called torticollis, a spasm with scarring of the sternomastoid muscle that requires time and sometime PT to correct, and rarely surgery. It could also be enlarged (reactive) lymph nodes, in response to an infection. Based on the diagnosis made therapy will be planned. Good luck with his therapy. Regards.