For reference, I have four kids - mix of girls and boys. The oldest three are toilet trained (most of the time at ages 4-9) the toddler isn't. The oldest two can poop in the woods.

The problem is probably that you and your wife(?) have taught him to go to the bathroom in one way in one place. That's a hard skill to teach so you've been successful.

But he's two or three years old. Three year olds are very litteral and often don't deal with change well.

So my advice is don't worry about it. So long as he isn't having (many) accidents and you don't mind carrying a small trowel, you're pretty sure he won't decide the corner of his closet is a bathroom. And he'll naturally grow to be more flexible and tolerant of different foods and circumstances so long as you continue to introduce them to him in a positive, non threatening way.

And think of the story you'll have to tell as he gets older!! Did you get pictures of the toilet to show him when he's in Elementary school?