So we have increased our budget

[brian]

During the first campaigns. Pay him flat 5%.

You can increase it but don't increase it more than 1 or 2 %. Tell him if he managed to reach the favorable ROI then only you will think of increasing the commission otherwise not. And also you have hired a PPC Manager that doesn't mean that your job id done here.

Keep an eye on him. Track him. Tell him to show reports at the end of each day. We basically do what is being a pain in his a** until we got our ROI or get the job done. I hope that you get it right.

Nothing but the truth.

To OP I say don't pay in advance. Pay as the campaign proceeds. If you see any sign of fault stop the campaign immediately.