That is a very good question. I would think the % should be based on how much work is involved in him getting you these jobs.
If the average lawn care business spends 5% to 10% of their income on marketing. How about something around there as a referral for him?
Although this isn't exactly the same situation here is a quote from this GopherHaul blog article.
“In a situation like that, they would fill out an estimate request sheet in their daily route book with their name on it. My secretary sets up the estimate. Upon receiving complete payment, he or she get their bonus. It ranges between 2 and 4% of the total job not including tax. So if it’s a $10,000 job, he can stand to make up to $400.00. Not bad huh?? And it’s all factored in to the numbers of the estimate so it doesn’t cost me a dime. For lawn estimates, they get the amount of the first cut as a bonus.”