16 mon. Old son has frequent bouts of prolonged sticky foul smelling stool, and trouble digesting any milk including toddler formula. Why is this?

See below. There are a number of possibilities for the problems. The best rec is to see your doc and discuss this and if need be, see a peds GI specialist.
Lactose intolerance. He may have lactose intolerance. You can try lactose free milk and diet for a week and see how he does. This is assuming that he is otherwise healthy and gaining weight in a healthy manner. If he does not respond or has other problems, he needs to f/u with his doctor.