Give your co-founder (1/48)(50% equity)(# of months working on the mvp). This is basically saying that your co-founders 50% stake vests monthly over 4 years. If they've worked 4 months on the mvp, 4 months worth of equity is released to them. If they leave, the remaining shares go back to the company.

For future reference, you really should have figured out the arrangement with your co-founder prior to working on ur mvp.