If you gift someone shares they have to count them as income. On day 1 when you valuation is arguably $0 this is NBA. 50% of $0 = $0 but today 10% of your business at a conservative 5x multiple is closer to $200k. Might be more, might be less, but that's income and your partner needs to pay taxes on it.
This could be the worst/best gift they receive. Typically what you're describing is done with stock options. That is, granting him the right to buy shares at a future date at a fixed (low price). No matter what people tell you options != equity. You can approximate what the right number to give him are but it'll never be the same as having 50/50. If you want to give him 50% you should issues shares, but make sure you give him a big cash payout (hint: also taxed!) so he doesn't end up underwater.