Don't have US phone number

You probably want to find a better way to get an iPhone. From their warranty document for i{Phones and iPads:

That means that if you need warranty service of any kind, it will cost you a plane ticket yo bring the phone back to the US.

Order one from your own country and give it to him before he leaves. If he buys one himself while in the USA, he'll likely be denied warranty service internationally, and need either a roaming plan or a local carrier which provides eSIM support.

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