"He gave us a lot of work outside the scope of our monthly agreement"

Which you don't do until he signs off acknowledging it is Outside the Scope and agrees to be billed as an Additional task on an As Requested scale.

Or, it is becoming routine and it's time to add it as Additional Scope = a Change Order with the new additional service fee.

"We also have separate monthly Bookkeeping agreements to review all his accounting work on monthly basis. He think that with that monthly agreement it should cover everything ."

I treat these as, "If you want to do your own work and agree to follow our directives, then the time it takes to do this naturally goes down, and the service fees for these also go down, by itself." Because he is learning what it is supposed to be, the first time it is handled.

Or, "When we do the review of our own work, we don't charge extra for our own follow up. But if we are reviewing work we did not perform, this is charged separately. All of this saves time in the tax prep cycle; you would pay for that at one stage or the other."

Because there is often not enough time to do it right, but always time to do it over.

It really depends on the Client's participation in the development of the process. I have some clients that have almost nothing to fix by year end. And I have some clients that keep trying to "slip one past me."

And here's a big Value Added perspective: look at how the tax regulations keep changing, nearly daily right now. Point out that you are the ones staying current on this topic, but he should focus on running a successful business, and not try to take on both responsibilities.