If his only responsibilty is to help you than a percentage would be the best way to go. As long as you are able to get him to do quality work and not cut corners.
The main issue with paying percentage is they tend to not take the time to do it right.
The main issue with paying by the hour is they tend to drag a job out especially if you have a light day.
BTW, Is There A Reason Why All Your Words Are Capitalized?