So here's the thing: Ideas are cheap. It's the execution that matters. It doesn't sound like your project has a lot in the way of execution right now, so if you were going to get help on it, now would be the time.

Even if he steals the idea, he'd still have to execute on it.

If you're wary, you can make a contract.

I know he said he'd be okay getting nothing except experience, but I'd personally feel weird about that. I'd come up with some type of payment structure (or even equity) once you start monetizing. That way you don't have to pay before it's monetized, but you're also honoring his contribution.

Again, that's something to put in a contract.