If the bitcoins are given in exchange for work, he is taxed as if it was income, and at income tax rates, much like if he was given a stock grant in a public co

If the bitcoins are given in exchange for work, he is taxed as if it was income, and at income tax rates, much like if he was given a stock grant in a public company. Also, these taxes are calculated when he is paid, not when he sells the bitcoins, so if they lose value, he is still on the hook for the full income tax (which is in USD in the US). However, if he sells bitcoins in the future, he will be forced to pay capital gains taxes on the difference in price between when he received the bitcoins (and paid income tax then) and when he sells. If he sells at a loss, he can use his losses to offset his income tax.