You could tell him that you spent X hours on the original design and implementation, and you charged him Y by the hour, or if this was a flat fee or cost-plus show him what you actually earned for each hour spent on the project.
Then give him a small allowance of 5% or 10% of the original price and tell him that the after-the-fact charges are a courtesy to him but if he requests changes after that allowance is reached, the meter is ticking again. You also try to raise the hourly using the fact that the work of doing these changes is intermiitent and not part of a much larger project and consequently your rate must be higher.
If you give him a little wiggle room to make SOME changes, you are respecting the customer and accomodating him. But you must place a limit on this, otherwise you will be trying out this or that or something he saw on CNN or Fluther or Yahoo and you will be dragged into a no-profit situation.
As a matter of fact, if you do begin to charge for the overages, bill him for a block of time upfront and make sure it is paid within 10 days and before you spend any serious time on his project.
Just some thoughts.
And the posters above have given you excellent advice.
SRM