Do the math on what x% is in dollars based on your premoney evaluation. Give him the note for that amount and treat him as if he were a friends and family investor.
If you don't have a premoney, then cut him a fair salary for the time he worked in a convertible note. The convertible note will be the optimal way to have a nice looking cap table.
I just suggest that you deal with this matter quickly while everyone is in a good mood. Success in a business after someone leaves can be dangerous...