In your scenario he would owe tax on the entire $75 gain. At the time of the gift he acquired your original cost basis of $100. His capital gain would be long term (and taxed at a lower rate) if he sold the stock more than a year after your original vesting date. (Note that the 2019 long term capital gains tax rate is 0% for single taxpayers whose taxable income is $39,375 or less.)
If he sold the stock less than a year after your original vesting date, his gain would be taxed as ordinary income according to his tax bracket.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
