Another option.
Rather than worry about the ethical rigmarole of buying at his retail, then marking up. How about this:
Offer him 80% of his price, to buy bulk quantity. Mark up 50% above his original price, then sell on his behalf, splitting the profit 50/50.
Hell be making 25% over, you'll be making 25% & if you split the pitch fee (if applicable) & transport costs, you could actually end up making a really good friend through business. Permission through mutual benefit will probably end up getting you preferential treatment, first chance at choice crops & possibly a chance at influencing his growing choices. You could also work in ways to deal directly with local restaurants & Whole-Food type local source markets.
If he rejects your plan, shitcan the whole idea.
Good neighbors who have a constant food surplus & generous nature are absolutely irreplaceable.