You should always give someone what they deserve. Never more and never less.
Most people don't know how to do this so they guess. They try to predict the future or they look for rules of thumb or they try other ways of guessing. Kind of like you are doing now.
The best way to determine this is to consider one person's risk relative to others.
When someone contributes to a startup company and doesn't get paid they are accepting risk. The value of that risk is equal to the fair market value of the contribution they made. For instance, if you could earn $100,000 a year doing whatever it is that you do and you do it for a startup without getting paid you are, in effect, risking $100,000 a year.
Taking risk in a startup company is essentially betting on the future outcome of the startup. If you and I bet $10 on the same hand of Blackjack we are each betting the same amount and, therefore, each deserve exactly half the winnings (if any).
So, the right way to split equity in a startup company is to keep track of what's been contributed, then perform this simple formula:
Individual Ownership (%) = Individual Risk/Cumulative Risk
The model changes over time as more contributions are made. Each day a person contribute their stake would change. This means that at any given time, no matter what changes, who joins or who leaves. Everyone always has exactly the ownership they deserve to have.
Unlike traditional models that require us to predict the future, the relative risk model is based on easily observable values in the market. Everything has a fair market value.
So, the answer to your question is simple. Add up the risk he has taken and divided it by all the risk taken by everyone (including you). Each person's share can be calculated this way and the total will always equal 100%.
On day one, before he's done anything, his ownership will be 0%. As it should be. Over time, as he risks, his % will change based on relative risk.
This is a perfect, unambiguous formula. Every other equity model lays the foundation for disputes later on. Only a relative risk model will give you the fair answer.
I've written a book on this topic, called Slicing Pie, you may have a copy if you contact me through Clarity.fm or SlicingPie.com.