First off, change the mindset... you dont need the client bad, they need YOU bad. Walk in with confidence, with your homework done, your facts straight, and knock em dead. Hot chicks act like they could care less, and get all the guys wagging their tails like lost puppies at them... fat chicks act desperate and get zilch. Be the hot chick, he needs you and YOU have to decide whether you want him as a client or not. Now, on to the other things...
Do your homework. How much traffic does the site get monthly? What keywords do they already rank for organically? Do they do any PPC? (All info u can get from the freebie version of SEMrush). What's their website like? Do they properly use title tags, meta tags, etc? How well does their site grade against the Top 18 ranking factors? (see SEO Master Certification)
What keywords are they not ranking for that they should? What is traffic look like for those unclaimed yet highly relevant keywords? I was surprised the other day to find out that the phrase 'armoured -----' outranked the phrase 'armored -----' in monthly searches, and its just because of the way Brits spell it vs. Americans.
Do you know what their typical, average sale is? Do you know on average, how many sales per week or month he makes? Do you know, based on his industry averages, what his average markup is? How long on average, does a typical customer stay with him - months? years? How many leads does it take him to get 1 conversion? How long does it take? This information is worth pure gold to you, and him. Pure gold.
Do the research, come armed with facts, and blow it out of the water. He needs you, not the other way around - and dont you forget it.