Grocery shopping with children is SUCH an ordeal! My son is six, and I still try to plan shopping for days when he has an after school activity and I can get to the store and back before he gets home.
My suggestions (in no particular order)
It really helps to make a list for yourself. Then, if you can, organize the list by section of the store so that you don't end up doubling back or missing things on your list. (most stores have online list makers or isle maps if you look for them it is a huge help). That will help YOU in the store and make it easier on you both.
Make a list for DS. He can help you (er boss you) as you shop WITHOUT getting out of the cart. It'll take some set up time on the front end, but once you have all the pictures in clipart, it's just a mater of dragging them onto your list and printing before you leave.
Also, bring him something else to do. A book, or photo album, or one of those lace up puzzles, etch-a-sketch etc. (Just bring ONE thing though or you'll end up with a whole extra problem on your hand and he switches activities every 30 seconds).
Be quick. Seriously. You'll be happier going twice a week if you can get in and out quickly than going once a week but taking forever and dealing with your son getting bored.
HTH
T.