How do I re-negotiate the terms of my startup?

I've invested ~100 hours in the app idea of a family friend, collaborating with him very loosely as I've developed. His central idea is great, but when we meet, he spends most of his time talking about bolted-on features, such as survey-based advertising, cloud storage, and celebrity promotion, which would completely trash the app's cohesion and user experience. So what I've been developing is essentially the central 20% of his idea.

This friend has no business or technical experience, and works full time as a (successful) manual laborer. What this means is that if I want the app to go anywhere, I will be in charge of financing, advertising, and recruiting customers, not to mention 100% of the development itself.

This friend is a very down-to-earth guy with six kids, and I respect him, but his initial vision of 50-50 co-ownership is obviously not going to work. Since he won't be contributing to the growth or operation of our startup, I feel like it's fair for him to own maybe 10-20% of, it as collateral for the initial idea.

Is this reasonable of me to conclude? How should I confront him about this?