That's so cool you are looking into getting a phone for your son, @hur ! He is either very lucky, or very good. Either way, a new phone is an exciting purchase, and an even more exciting gift to receive.
For whatever reason, it seems pretty much impossible to use any but a card issued by financial institutions based in the US, and with a US address, to customers registered with a US address on that card. Which doesn't mean that there's not a way, only that if there is one - I don't know of it, and trust me I've tried.
If you discover the way around this, I hope you'll return to share that knowledge.
For myself, I've pretty much accepted that there will not be a way around this restriction at this time.
Manage your T-Mobile account