For a technical co-founder, the minimum should be 33%. If you've been paying him, there is no way he should be 50/50 but how to determine the exact number is a calculation of a lot of variables that are impossible to answer without knowing more about your situation.

Most investors want to see a team in place (especially a strong technical team) so incentivizing the right person now might make a lot of sense.

What is standard is for all employees (yourself included) to be vesting over 4 years, with a one-year cliff. Given that you have invested cash, that should be treated differently (not subjected to any vesting) but your "earned" equity should be vesting just the same as this developer.

BUT, keep in mind that if you are only a month away from launching, you might be best to continue investing cash. After a product is validated (assuming this takes place), the amount of equity a developer should expect to earn could be significantly less than 33% and even single-digit percentages.

Happy to talk through the specifics of your situation in a call.