FD, sounds like such a nice project to do with your son! I don’t have any experience with sourdough, so sorry, I’m no help there.
I do have an eight year old daughter who bakes with me, my best advice on that count would be to allow him to take the lead where appropriate. He can choose his own loaf shape and discover how much it changes as it rises. He can solve problems with you (where do you think we should let it rise? How will we know it’s doubled?) and definitely expect him to make more of a mess than you do! Have some of his favorite bread toppings on hand, or maybe even a new, special one, for the tasting. If I know kids are going to bake with me, I try to get a good night’s sleep the night before so I’m at my most patient, tolerant, good-mommy best.
“Sourdough Sunday”- sounds like it could be a regular feature! The kind of thing kids grow up with and then do with their own kids years later.