On the U.S. TV sit-com "Mork & Mindy", who played Mork's very large baby?

On the ABC U.S. TV sit-com "Mork & Mindy" (1978 - 1982), the person who played Monk's very large baby was Jonathan Winters. Mearth (Jonathan Winters) was the "child" of Mork and Mindy and godson of Orson. Because of Orkan physiology, Orkans would age backwards, starting with elderly adult bodies. The bodies had the mind of a child, regressing to feeble "old" kids.

"Mork & Mindy" was a spin-off from the hit show "Happy Days." In the show, Mork was played by Robin Williams. He was a lovable alien from the planet Ork. He had been sent to Earth by his fellow Orkans who saw him as a misfit; his assignment was to observe the customs of the crazy Earthlings. Landing in a giant eggshell in Boulder, CO, Mork is befriended by Mindy McConnell (Pam Dawber) a music store clerk. He is allowed to stay in the attic of her apartment.

Eventually, the relationship between Mork and Mindy jumped from a strong friendship to romance. They get married and have a son from Earth named Mearth. He isn't quite like his dad!