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This is likely a swollen lymph node.
Seasonal allergies can cause lymph node inflammation commonly.
Also recent viral infection can cause this as well.
Another possibility can be a fibrous cyst --a benign condition.
Bone cyst is another possibility from collar bone head also benign.
Without any other symptoms this is not usually clinically significant.
However, this should be checked by his doctor also to confirm what i said above ( I am unable to examine online).
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