He has a huge knot behind his ear by the jaw bone and he is tired?
Swollen Lymph Node
Eugene - Of course I cannot be sure without examining your son, but I have a question or you since the jaw bone is not behind the ear but in front of the ear. If the know is behind the ear lobe and below the ear then that might be a swollen lymph node.
That usually happens due to a bacterial or virus infection.
MY suggestion is to take your son to his pediatrician.