He can bring as much as he wants without having to pay anything. He is required to declare any amount over $10,000 but nothing happens if he's bringing in more than that, they just want to know.
Getting on my flight to Nigeria in April from New York (JFK) they asked how much money I had with me. I told them the truth ($4500 in US Dollars and UK Pounds) and he said I only had to declare if I had over $10,000. He then pulled me aside and went through my purse and my carry-on, and patted me down to see if I was lying! He even opened sealed envelopes which contained wedding cards. I was so upset. I am a Canadian citizen and this US customs agent was acting like I was some sort of criminal, after I had already declared and shown him a substantial amount of money, which I wasn't even required to do. Sheesh!