Ask HN: How to invite a silent partner into startup with significant equity?

My friend is a highly connected AND very technical salesperson. A year ago he approached me with a radical idea: there is a sea change happening in his industry that will force companies to abandon existing software infrastructure, and he had an idea that addresses this sea change in a way that does not force his customers to make billion dollar reinvestments in their networks and handles with a new suite of software instead.

I was working as a consultant and setup a company (using Stripe) and for the last year have been building the software. We are close to having several large banking customers do paid trials.The software works and is radically different than anything else out there, but it fits easily into existing solutions and the sales cycle is not alien.

My friend works for another company in the same industry. When he first had this idea, he told them about it, and they declined to act on it. He has this documented, by the way. It would have cannibalized their opportunities as a reseller of technologies that compete with this new offering.

I've taken zero investment up to this point. There is also no revenue yet.

My questions are:

1. He will leave his current employer soon. How do I let him buy into the company and purchase a large equity stake (for very little cash) without drawing the ire of the IRS if there is a later large sale?

2. He has some debt, so one thing he would like to do (and I'm 100% on board about) is letting him purchase stock at a very reduced rate now. Then, when we prove the opportunity and several of these banks do paid trials for $50k or so, the company buys back some of the stock from him (at a very different rate). This obviously establishes a very different valuation for the company, and allows him to reduce his personal debt.

Are there better ways to do this? If I offered him a position on the team (as a C-level executive), could I grant him a large equity stake (say 25%)?