Since DS2, age 3, had his measles jab he has dropped his daily afternoon nap, getting him to go to bed at night is a palava and he wakes in the night which he doesn't do unless he hasn't slept in the day.
Now I know most kids have given up napping at this age, but mine sleep until later than most, and this is annoying. His nap time was my time to MN eat lunch and get jobs done.
Why don't you try quiet time instead, goes to his room with some toys/books and a drink and has half an hour 'rest'. Worked with DS1 for a while and eased us over the 'shock' period anyway. You very quickly get used to them not napping, have lunch with him, let him have half an hour cbeebies or a dvd, give him a crafty project he can do fairly independently while you get on with stuff, get him to help you with whatever you're doing (DS1 is a mean duster)