The BB, because of the heat associated with firing, may not be grossly contaminated. But, infection is a possibility. Besides, the BB would have transferred most of the microbes to the tissues through it passed & your son would need to take antibiotics anyway.
The other risks are migration which is often harmless. You can definitely pursue surgery, but I do not think it is necessary.
If infection or other reaction develops, surgery can be resorted to. But I would not have advised surgery in their absence .
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