Joshua "Josh" Gabriel Matthews is the third son of Alan and Amy Matthews. He was born with complications and after a lot of touch and go, baby Josh recovers and goes home with his parents. He's aged up in the series finale (played a producer's son) and gets some knowledge imparted onto him by his older brother, Cory.
Daniel Jacobs' appearance as Joshua in Brave New World severely alters the continuity of the show, as he should be much younger. This is a continuity error.
Joshua has a recurring role in the sequel series Girl Meets World, appearing as a teenager who has a close relationship with his older brother Cory and niece Riley. He's played by a different actor in Girl Meets World.