Normally you should put together a contract with a price for thumbnails,a sketch (or 2,3), final and 1 or 2 revisions (beeing small/medium adjustments to the final).
If he wants additional revisions he has to re-negotiate the price.
Normally standard prices for sketches, final pieces, revisions should be available online to calculate a total.
If you think these prices are to high to start with or to be competitive give him a reduction. If clients complain too much about money just refer to those standard prices.
If you don't have a contract just tell him you can't do that amount of work for the prices he's paying atm. He can negotiate a new price or you have to terminate the cooperation.
"Create like a god, command like a king, work like a slave."