Thanks all of you who posted suggestions for me! You see, not having grown up around smaller children or having had experience with little ones, even at my wise old age :), I just don't have many answers when it comes to children. All of you suggestions were great!
I took my son grocery shopping this morning, on a big shopping trip, EARLY. I figured that way we were bound to get a car cart. Sure enough, he got to choose between red and blue. I also let him bring some fun toys, and snuck some fruit snacks in my purse for later also. As well, one of you mentioned having a separate list for him. Great idea! I had him hold it, and as I collected those items, along with some others, he held them down in his car until checkout time.
Funny thing, as we started out by dairy, there was a rack of bananas and he turned to me to say, "Mommy I'm hungry. Want a banana!" So that kept him occupied for some time (just have to remember to pay for that extra banana at checkout. I forgot once and paid two weeks later!), and I saved the fruit snacks for the produce section, which has always been tempting to him.
It is a big start, and something I really want to keep up, so we have that quality time (if you can really call it that) together and it is no big deal to him too.