Actually small subcutaneous skin (under the skin) nodule is usually due to a neurofibroma (connective and nerve tissue), a ganglion cyst or a lymph node. Conditions causing skin granulomas should also be ruled out. Swimming pool granuloma should be looked into first if your son swims. This is caused by Mycobacterium marinum bacteria and the test is same as for tuberculosis. Apart from PPD tuberculin test, skin biopsy—the bacteria can also be cultured. Tick bite is one possibility and if you live in an area prone for tick bites, then Lyme's disease should be ruled out by appropriate tests. Pseudolymphoma is another possibility which causes persistent inflammatory nodules. The good thing is that they are usually without symptoms and disappear on their own by 8-9 months. These can be due to calcinosis or calcium deposits if you have received calcium supplements in the past. An autoimmune condition called granuloma annulare is another possibility and hence immunoglobulins and other tests for autoimmune disorders of which rheumatoid arthritis too is one should be looked into. Sarcoidosis, tuberculosis, pseudorheumatoid disease, foreign body granulomas, leprosies, leishmaniasis (parasitic disease caused by bite of sand fly), lupus, hypothyroidism, diabetes, and deep fungal infections should also be looked into. A ganglion can also undergo granulomatous change. Other tests that may help include serum calcium levels, immunoglobulins, blood sugar, thyroid profile, tests for lupus and Lyme's, and tests for autoimmune diseases which would also be a part of rheumatoid arthritis profile. Please discuss this with the skin specialist for further investigations if need be. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.
Thank you for your advice, very helpful when he has more tests. I will suggest som, if they don't.