Depending on how old he is, yes. Your son is probably at the age that he can make his own decisions. Trust him. But what you do, is up to you. Don't jump off the bridge because someone tells you to. Make the decision that you think is right.
Another opinion:
Suggest to him a speedo brief. They also can be a bit revealing but he won't be as naked looking as in a thong.
Another opinion:
Trust him to make the right decision. If he is wearing thong underwear, no big deal. Some dudes wear boxers, brief, boxerbriefs, some dudes wear a jockstrap as daily underwear and some "freeball". As far a a thong (swimwear), if he is a dude who is in shape, ok. If he is overweight, no.
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