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Mr. Jeffries is the son of Arthur Jeffries aka Professor Proton and his ex-wife Mrs. Jeffries.
He is mentioned in "The Proton Transmogrification" by Leonard when he tried to ask Sheldon if he knew that Arthur's son was a science something in high school, but Sheldon cut him off because he suddenly hugged Leonard. This is the line of the mention:
Leonard: "Did you know that Arthur’s son is a high school sci…?" (Sheldon hugs him)