Ahh. Well then, if it was me, I think I'd find a sweater and a pants pattern in worsted or bulky yarn (for speed's sake) and talk about how he will get out of his special suit if he needs to go pee. If he still wants a suit - just knit the pants onto the bottom of the sweater adding an extra inch or two (or be careful on your measurements to be sure the outfit will go from shoulder to ankle).
My 6 yr. old freaks out if he has to wear pants with a button - he waits till the last minute to go pee and is then in a panic trying to get out of the pants.
There are lots of
free patterns here.