When he appears as a child in 1970's Dimmsdale, he has a brown afro, thick coke bottle glasses, and a teal blue shirt. He speaks with a nasally voice like Elmer, which is not odd, because Billy shares the same voice actor as Elmer.
Billy Gates has appeared twice in the show. In "Father Time!", his first appearance, he is shown returning from traveling in time with younger Cosmo and Wanda from the 1940's just in time when Timmy was arriving in the 1970's. In this episode, after Cosmo and Wanda showing a computer of downloading lava lamps, he devised the idea of creating Internet. Cosmo suggests he name it "The Timmy" instead, and Billy takes his advice. When Timmy returned to the future, he found that Billy Gates ended up taking present Cosmo's advice and that the Internet was named after him (and Timmy was named "Internet").
In "Hassle in the Castle", he is shown in a portrait in the Hall of Fame in Cosmo and Wanda's castle, indicating that he was a good godchild.
In real life, Bill Gates is not really the inventor of the Internet, but in the Fairly OddParents universe he is erroneously referred to as such. Although he is the founder of the company Microsoft and an important pioneer in the creation of personal computers in reality, he had little to do with the creation of the Internet.
In the episode The Secret Origin of Denzel Crocker!, when Timmy traveled back to the 1970s Cosmo and Wanda are Crocker's fairy godparents so that means Cosmo and Wanda were either assigned to Bill after or before Crocker. But in Hassle In the Castle, Crocker's picture appeared first, so it could be possible Cosmo and Wanda were assigned to Billy after Crocker lost his godparents.
Dee Bradley Baker voices Billy Gates, who is listed in the credits to Father Time as "70's Kid".
In Father Time he is potrayed as a 10 year old in 1970 while the real life Bill Gates was actully 15 years old.