My suggestion is not about "cooking" really, but experimenting with texture and properties. In addition to cooking, which most kids love, how about setting up alternating tubs of dry beans, flour, rice, etc., for him to work with without your supervision. "Good" preschools (as opposed to sink holes where kids fill out worksheets) have sensory tubs filled with stuff like this as a matter of course. Children love to explore texture. Dumping and pouring are staples of early childhood experimentation.

Add some measuring cups, colanders, wooden spoons, etc., and he can stir and mix. One bag of flour can be experienced over and over. It's not really waste. A tub of water filled with eye droppers, turkey basters, funnels, spouted cups, a hand held egg beater, Brio water wheel, and pump, can be paradise for some kids. I have added shaving cream, glycerin, gentle dish soap, glitter, oil, food coloring etc. to water. All with magical results. A giant old towel under a small table, with the tub on top can be easily set up and put away. :-)

Laurie (Who has been a teacher in good schools as well as in sink holes.)