You can also strike a deal that can help you long term. You can do the work for nothing up front but get a % of the IMPROVEMENT you make to his cash flow on the backside and have income for years.
So if he is earning $100 profit from his site per month and after your SEO work he gets to $500 profit per month, the difference is $400 profit per month, even if you get just 10% of that that is $40 per month for as long as he runs the site. Over time as the profits increase, so do your royalty checks. You can also offer a buyout, if he does really well, say that his site gets to $5000 profit per month you can offer him a buyout of say $1000 so he does not have to pay you monthly anymore.
Those are just some ideas and the figures can vary.