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1. The history suggests that there is a
nerve compression which is causing the above symptoms.
2.The possible cause could be some trauma /
injury caused - he might have not noticed. Other possible cause infection / inflammation to the nerve.
3.The lump under arm / axillary region could be enlarged
lymph nodes. Enlarged lymph nodes are seen in case of any infection localised or systemic.
4.The next step should be - see your GP get examined, and probably
MRI elbow joint might be needed, for better evaluation of status of the nerve, tendon , joint space.
5. You have to take him to GP or any doctor as symptoms seems like nerve compression - needs attention and treatment. Good luck.
Hope it helps you. Wish you a good health.
Anything to ask ? do not hesitate. Thank you.