Revealing mistake: A different horse is used for close-ups and race shots; you can tell by looking at the white patch on Pie-O-My's head, which is much bigger in the close-ups.
Continuity mistake: While standing outside the horse's stall, Ralphie's arms go from being at his sides, to folded across his chest throughout the whole scene.
Answer: That's not Jimmy, just a look-a-like. Jimmy's short and stout, the guy in "Christopher" episode is tall and medium built. The hair is also different.