It's a trade off, whether to pay full commission or salary/commission.
What happens if he underbids a job? No commission? That wouldn't fly if he was on full commission. What about underbidding many jobs?
How extensive is your marketing? Are you getting calls? You're going to have to pay a higher commission rate, if he has to go door-to-door to drum up business. Door to door is much less productive that having pre arranged appointments.
If he is paid based on profitability of the job, does he have a say in who does the work? If you have more than one crew, a job gets bid based on the productiviyt of the 'better' crew and ends up being done by the 'slower' crew, does this affect his commission.
Will you do any form of tree work, or is his restricted in what type of work he can sell?
I think these are some questions you have to ask yourself before you start the hiring process.