Simply tell them that building a web business is a long term project that requires a lot of work and commitment. A quick fix is not going to do it. If he was launching a new soda business would he expect to be bigger than Coke or Pepsi in 6 months?
We find educating our customers is crucial for a successful longterm relationship.
We teach them about search engines, spiders, directories and pay per click. At first we were worried this may give them the tools to go out and do it on their own, but that has yet to happen and we have been doing this for just over 5 years. In fact, it really helps you to demonstrate value in your skills when they understand we aren't going to just push a magic button on our seo keyboard, and boom, #1.
Broaden the presentation beyond what we would consider, “optimization”. Present the facts as they are. You can assist in bringing traffic to his site via organic, (optimization) pay per click (paid advertising) and through affiliate networks (paying others to help drive the traffic). The reality is he should engage in as many forms as possible, and these days promising all kinds of returns on traditional SEO is very risky.