I don't recall that coming up as an issue for him. I don't see why that would be a problem. You can just explain that 8/8 is one whole, leaving 3/8. I don't see why that would be hard to demonstrate.

"These are two pies. Let me cut them into eight pieces each. Let's count to eleven pieces. That's one whole pie plus 3 parts of the second pie, so it's the same as 1 3/8."