UPDATE:
He ate a large mouse. Still can't believe it fit down his neck, but I guess that's what they do.
The next size up is a small rat, and I'm pretty sure that that's too big for him right now.
He still hasn't shed or had a bowel movement since I've started increasing the size of his food.
I'll stick with the large mouse unless someone wiser than I thinks that that still isn't enough.
Hoping to get his first shed soon.