Speaking from past experiences as the former young guy. The relationship timeline should be at least a year as many strategies he implements will take hold over time, SEO, press releases, relationships with blogs, videos, social media, ect.
If he is to be your lone driving force I wouldn't worry about where the sales came from (meaning no commission codes or linking). Small numbered examples below:
Before him: averaged 10 sales a month
Month 1 with him: 16 sales
Pay him commission on 6 sales.
Use a scaling commission chart example:
First 5 sales 10% commission
Next 10 sales 20% commission
Above 25 sales 25% commission
Have defined "Projects" timelines:
Website redesigns: Completed before week 4
Facebook Page Redesign: Completed before week 4
Articles written per month
Hope that helped. Ideally the young guy should structure the deal and have you adjust it. It's hard to give specific commission numbers without knowing the costs, current sales and expectations.