Jamie is the boyfriend of Russell's mom, mentioned in s2e11 Dances with Wolfs Adventure.
He is mentioned again in s2e17 Dead Moon Walking Adventure in relation to a boxing gym.
But he's in Salt Lake City for the Toot-a-Loo Flute Festival.
A boy with black hair mentions him while standing next to another boy with brown hair.
Russell says "I could be his flute caddy" when mentioning he and his brother and mom could go next time.
He is presumably the blonde man shown playing a flute during the end credits when Russell sings.