Hi Michael, As a novice gardener myself with a 4 year old, (don't worry, the experts will chime in) I suggest planting whatever sounds good to you and your son. Things that are usually easy to grow are tomatoes, squash, melons, berries and beans. Smaller varieties of tomatoes are fun to eat right off the vine. Strawberries are fun to pick & eat. Herbs can be fun and easy, too - basil, parsley, thyme. I let my daughter make some choices of her own and help with everything. You might not get perfect rows, but your son will hopefully have fun. Sprinkling in a few flowers
to bring ladybugs, bees, and butterflies is exciting and great for the health of your garden.
Check out your local university extention or garden clubs or home/garden stores for advice on what grows well locally, as well as for workshops to help you get started. If you garden organically, you can be sure that there won't be any chemicals to worry about with your son. There are tons of great books and websites for gardening with kids.
You may also want to start a compost bin or pile. This will keep you from having to buy or haul in a lot of extra fertilizer or mulch.
If you wanted to, you could take photos of before, during and after and your son could help with a journal, or just keep the photos to see your progress. It's helpful to keep track of what you plant so you know what is coming up where and what to plant next time. Have fun and enjoy the process. You can post photos here so we can see, too, if you want.