My Son Is Having Problems With The Right Side Of

My son is having problems with the right side of his neck/ throat. He is 21 years old and for a while now at times he feels like his throat "pops out" to the side. When this happens there is a visible lump there and he has extreme pain and a hard time swallowing. It seems to go back in place after he swallows a few times. This is happening more freuquently and is very disconcerting.