My son wears about a 16 in jeans, and a small in men’s shirts, what sz sports coat do you think he would wear?

We are going on a cruise in the spring and he wants to take a sports coat/blazer with him and I was looking for one on e-bay. He is about average, not thick, not skinny, average height for a 12 yr old.
I was going to look in Mens wear since there are more available.
I cant seem to find one that is juniors, so I am looking at Men’s sizes

You need to measure his chest. Most sport coats are sized based on chest size and being either Regular or Long.

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