Wow, you've got your work cut out for you on your first client! It's a great way to start though. You will do such a great job for this guy that hopefully he will tell all of his colleagues.
The first thing you should do is get the current billing out and out correctly. It is important that you break the cycle of incorrect billing.
As far as the old stuff that was billed incorrectly, I would pick some guidelines to go by and stick to them. For example, I would definitely go back and resubmit any claims that were not paid. I would resubmit them the way they should have been billed in the first place. If they were denied for no modifier, or some other reason, I would correct it and resubmit with the correct billed amounts. If a claim was not paid, I would also resubmit it with correct billed amounts.
But if a claim was already processed and paid with incorrect billed amounts, I don't think I would resubmit for reprocessing. Having claims reprocessed that have already paid out gets messy from both ends. Resubmitting claims that were denied is no big deal. From the sounds of it, not too many got paid anyway.
This is really a great opportunity for you. You are going to change this Dr's income immediately.
Good luck!
Michele