Hi, thanks for sharing your son s problem with HCM! Well, not to worry about this and if I were his family physician in this case, I would just think about a retention-type of cyst or in worse case a hemangioma/lymphangioma type of swelling! In fact, thee is some loose collection within its cavity and it is very prone to get infected. I would like to suggest to confirm the diagnosis first by FNAC test/ultra-sonography and accordingly go for treatment as suggested by your surgeon! The treatment could be just aspiration of its contents or excision or just wait and watch under conservative treatments! But never it is some serious condition! See you again with feedback and all the best...