If I were you i'd make it simple so it makes sense to non-IT types. Just ask him to pay you per hour what the IT guy at your business gets paid. For every hour you put into it, thats how much you should get paid for it. Also, don't count the time where work isn't busy as free time to work on his crap, you need to add that in there too. Whenever you are working on it thats billed time simply put.

Its cool that you started working on it for free and did extra stuff here and there, you showed your worth. But now that you have made your services known and respected (at least at your place of work) you need to be compensated.