The lumps in his neck are likely to be lymph glands.
Lymph glands can get swollen in many conditions- most common being acute infection like a sore throat
, others being chronic infection like Tuberculosis
, connective tissue
diseases, malignancies etc.
Since you say he has fatigue etc, he needs to be evaluated fully.
A complete look into his symptoms (like fever, cough, sore throat, any similar swellings in the body, rash, joint symptoms etc) and a detailed examination will be needed.
Based on the examination findings, basic blood tests will be needed- like a complete blood count
, peripheral smear, ESR etc.
Your doctor would have ordered for Ultrasound
to look for any other similar swellings inside his tummy or to look for his liver and spleen.
Please ask your doctor to provide you the needed information. They will surely tell you why they have ordered for specific investigations.
Its difficult to say what the exact problem is without the needed details and without examining him.
Hope my answer helps you.
Wish your son good health.