We started with a banana. I demonstrated myself, then assisted him with the motion.
He seemed to find the feel of the knife cutting through the banana interesting.
Next we tried a soft pear. I cut into the pear a bit first, then let him put the knife in and continue the cut.
Due to some sensory quirks, he was reluctant to touch the juicy pear to stabilize it with his other hand, but he did manage to further my cut and get the idea.
We've also practiced cutting using play-doh, which he enjoys as it's much less messy. Many also start with a block of soft cheese, though Ty is sensitive to dairy. I'm quite tempted to buy him the Small World Living Toys Peel and Cut Fruit or veggies. His favorite part about going grocery shopping is running around to touch and explore everything in the produce section, so I think he'd enjoy these for two reasons. I'm trying to cut down on what I buy for him, but I just might talk myself into this one!