Edit: for grammar
I turned down an offer once to receive 1% or profit on every transaction that came in through some web-services I build in lieu of cash up front. It was the worse mistake I ever made. That service did over 50 mill in profit the first year alone. I should have done the math and took the risk. So it all depends.
A startup company or startup is a company or temporary organization designed
to search for a repeatable and scalable business model. These companies,
generally newly created, are in a phase of development and research for markets.
The term became popular internationally during the dot-com bubble when a great
number of dot-com companies were founded.
It's much more difficult to get right when you are building long-term assets (such as a website) instead of making one-off sales. If you build a website and leave, do you keep getting revenue share from it forever? If the company branches out to a new field, is that included?
But the fact that it's not a scam might not mean it's a good deal. Do an estimate of the cost of the project considering your rate at your job and the time the project will take.
Compare it with what you're expect to make with the 5% in the next few years.
Maybe they are added benefit of doing the project with him, like experience with a new tech or having first client of a side business making web apps.