Hi Richard. My humble 2 cents would be to ask yourself...how much is he investing otherwise? How much time, sweat equity, and risk is he putting in? What is he giving up or sacrificing to do this with you? How much time away from his family, friends, life and even day job is he giving to this project? How invested is he emotionally? Does he feel as invested as you; is this project his baby, too? Is he in this for the long game like you are? Is he as passionate about the idea, regardless of the outcome? Does he want a leadership role; has he hinted or explicitly stated as to what role he envisions playing in your company? Has he seen the end game, talked about what he imagines the outcome of your work together will be? Does that vision align with yours? Most importantly, how much work is he putting in, and is it on a consistent and reliable basis? What would he ordinarily be paid for such work?
These are just some questions I'd ask myself about a co-founder, to see how invested they are and to also determine a level of appreciable equity. I'd see their work ethic, quality of work, attitude, feeling, team work, etc.
I'm not putting a number out there for you, because I don't know your scenario intimately. As for me, I have a very lean team for both of my startups. It's just me and my co-founder. And I'm willing to give 50% (vested over an agreed-upon schedule) to someone whose got just as much skin and effort in the game as I do, even if they haven't put in the money like I have. To me, it's more important that they feel like this is their baby as much as it is mine. When that is the case, I've noticed that two good parents emerge -- both feel equally responsible for the outcome of raising this company and both are equally invested to make the same gains when things pan out. Once that is covered, there is no feeling of superiority from anyone in the team/duo, no resentment from the person who has less equity, and no lack of effort due to assigning more responsibility to the person who's got a bigger percentage of the company.
I'm sure I'll have dissenters. I'm not giving you advice, as much as I am showing you how I've treated your question in my personal life, in my own way. Best of luck to you!! :)