If you are going to be submitting the billing on his behalf the normal fee is to charge a percentage. The average is from 5% - 13% depending on where you are located (in the country), what type of provider, and what level of service you are providing.
You could charge a flat fee but I think it would be hard to get enough info to determine what that should be. You could also charge a per claim fee, but if you need to track the claims that doesn't always work out well.
Michele