Need help deciding vesting schedule / cliff for co-founder

Hello, I've been working on my startup for around 6-7 months. I'm the founder / CEO / CTO of the org. I have a question regarding vesting and cliff for my co-founder who had recently joined around 2 weeks ago. The co-founder is mostly responsible for Sales, Marketing, Revenue. We are both fresh out of grad school , I've got the technical background for the most part and he has the sales background.

Background info about the company:
We have around 15 users so far, some logged on to the app, tried a few things, and logged off and most likely did not return. (Acquired mostly through cold outreach, no ads or proper campaign is set-up yet)
We have zero MRR.
I have built an MVP and am working on improving it

Some of the tools i built are not necessarily as important to the users as I imagined, and I am working re-working those into more useful tools (most of which I will be handling) after acquiring feedback.

We agreed on a 37.5 / 62.5 split. I have got 62.5.

My question is how should a cliff / vesting schedule look like for the org? My co-founder wants it to vest immediately, I'm not sure if this is the right decision but i don't want him to feel like he is being used for work or give off an employee - boss vibe. But I want to be careful and safe with the decisions I make.

I was thinking something along the lines of a 3-6 month cliff with a 1 year vesting schedule, and if we sell during that time I will be fair and give some more equity to account for in general work being done (I am not sure how much more). My co-founder suggests that it vests immediately and if he leaves then the equity goes back to me.

Bottom line, I don't want to be irrational when splitting equity and vesting in the company, and I want to make it fair for both of us so I need some help deciding a vesting / cliff schedule for myself and my co-founder. Right now I haven't set up any cliff / vesting schedule for either my own equity or his.