In most cases, lymphoma is a painless condition. Swelling under the chin can be caused by enlarged lymph nodes that are responding to nearby infection or inflamation (or, sometimes, this can also be due to inflammation of the salivary glands due to blockage of the duct, or tube, that drains saliva from the gland).
As I cannot examine your son, I'm afraid that I cannot offer you (or him) a specific diagnosis (or specific medical advice). You should, therefore, take him to see his doctor for a proper evaluation.
Sincerely/ Robert A. Wascher, MD, FACS
www.doctorwascher.com
www.oncoguard.com