These lumps, I guess, may be enlarged lymph
nodes. If the size of individual swelling is not > 1 cm (in diameter), and if your son is not sick, then you need not worry. Such enlarged lymph nodes without any significant disease in them are frequently noted in children. The exact time (/ age) of their disappearance can't be predicted though it can be expected anytime from now onward. At present, I would recommend only observation.