You don't see him on screen here, just a voice in the crowd that's singing the song. It's 1970, a recording session for the theme song for the upcoming 1971 series "The Electric company".
36 years later, he shows up on a famously bad UPN show, does a good job, basically the exact same acting and character that made him famous eventually, but gets thrown under the bus by a dimwitted team of writers and producers. No one really knows who he is yet.
Finally in 2011, he was hired onto a show with talented writers and producers, and became a household name. Since then, just years before retirement age, he is finally being given credit for his 50 years of work at acting. In the decade since, he's made enough money to justify all of it, and validated his entire career. Still, imagine getting a part on a national tv show, and then waiting 41 years for anyone to learn your name for the first time.