There are many reasons for a boy to have a lump on the left side of his neck, such as anterior cervical lymphadenitis, mumps, boils or thyroid disease. 1. Anterior cervical lymphadenitis: Inflammation in the head and face can return along the lymph nodes, causing anterior cervical lymphadenitis, which can lead to swollen and painful lymph nodes, causing a lump on the left side of a boy’s neck. 2. Mumps: caused by the mumps virus infection, when it causes inflammation of the submandibular gland, it will make the jaw in front of the neck visibly enlarged, resulting in a lump on the left side of the boy’s neck. 3. Boils: purulent infections of multiple hair follicles and their surrounding tissues can also cause hard knots, lumps, and pain under the skin in the early stages, which can lead to a lump on the left side of a boy’s neck. 4. Thyroid disease: common such as thyroiditis, thyroid cysts, thyroid tumors, etc., can also appear in the phenomenon of lumps on the left side of the boy’s neck. There may be other reasons for a lump on the left side of the neck of a boy’s head, so you need to go to the hospital in time and follow the doctor’s instructions for treatment.