I found this video of William Christopher, from 1985 I believe talking about his autistic son. This is the oldest clip of a celebrity speaking about their autistic child I have found. I believe the date is correct because he references the recent split of Sylvester Stallone's first marriage, which took place in late 1984 and finalized in a divorce in February of 1985.
William Christopher (born October 20, 1932) is an American actor who is best known for playing Father Mulcahy on the television series M*A*S*H and Private Lester Hummel on Gomer Pyle, U.S.M.C.
In 1985 he and his wife wrote Mixed Blessings, a book about their experiences raising Ned.