Sign away IP. You have a NextJS app. It's not worth anything there. But you did build the MVP for free (I'm assuming you didn't bill him) which is worth something.

Given that you won't be doing anything else after August. Ask for 10-20% instantly vested, or a combination of instant and scheduled. And, he has the right to buy you out in X amount of time at Y valuation i.e. 3x TTM sales.

So if the business were at $1 million sales in 2 years, he/his investors have the option to buy out your 10% for $300k. For example.

Don't do profit sharing on a pre-rev company. Pulling money out of small software company that's just getting started hurts both your interests.