WASHINGTON (AP) _ Sen. Lowell P. Weicker Jr., a millionaire several times over, can well afford the best schools for his children, including 9-year-old Sonny, born with Down's syndrome.

As far as Weicker is concerned, the best for his son means attending public school.

''I could afford to go ahead and send him to a private school,'' said the Republican senator from Connecticut, but ''there's nothing I know of that's as good as where he is,'' at the Woodley Hills School near the Weicker home in Fairfax County, Va.

''He just gets a first-rate education. He's in a (special) class of roughly eight or nine. It sits right in the middle of the school,'' said Weicker. ''He gets pulled out to go to a regular class for his reading. Sonny loves to read.''

Lunch, athletics and other activities are also in the mainstream.

It's a far cry from ''when all of us grew up and you'd never see a retarded child in school,'' said Weicker.

At home, Sonny is in the mainstream of family life with his twin, Mason, and two younger brothers. Weicker and his wife, Claudia, also have three grown sons.

''He's not sitting off somewhere by himself while the three of them are doing their thing,'' said the father. ''He can pretty darn well converse and read and watch television and movies and play sports. ... That's testimony to me as to exactly what's going on.''

Congress has no greater champion of the disabled than Weicker. From 1981 to 1987, he chaired both the Senate authorization and appropriations subcommittees dealing with the handicapped, giving him extraordinary control over programs and pursestrings.

He stood as a one-man brick wall against President Reagan's attempt in 1981 to cut spending for education of the handicapped and relax federal regulations.

When U.S. Secretary of Education William J. Bennett hired a policy aide who had accused handicapped advocates of being ''selfish,'' Weicker demanded and got her resignation.

When Nancy Reagan held a party at the White House to celebrate the 10th anniversary of the Education for All Handicapped Children Act, Weicker boycotted the ceremony and denounced it as ''hypocrisy.''

Weicker calls the law ''the greatest piece of legislation that has ever been passed in my lifetime.''

But Weicker said the disabled face a constant threat of losing hard-won services. ''I fear a backlash all the time, because they don't vote. It's as simple as that. They're easy to pick on.''