If he's a good friend, he no doubt knows about your startup/venture already. If not, start by giving him the details and presenting some of your challenges.
I'm guessing that he will have lots of ideas and solutions, and will even follow up after your conversation with more thoughts.
If he does, then mention that you'd love to explore him coming aboard. Seems to me you should be selling your vision and getting _him_ to bite.
If he does, then mention that you'd love to explore him coming aboard. Seems to me you should be selling your vision and getting _him_ to bite.