A man who says he is convinced that his son, shot down in a bombing mission over Laos 11 years ago, is still alive and a prisoner of war, filed suit in Alexandria yesterday in an effort to prevent military officials from declaring him dead.

Henry V. Fors, 63, a poultry farmer from Puyallup, Wash., named former Navy Secretary Graham Claytor and Marine Corps Commandant Louis H. Wilson as defendants in the suit filed in U. S. District Court. Fors wants to prevent officials from changing the status of his son, Marine Corps Maj. Gary Fors, from missing in action to dead.

Earlier this week, Judge Albert V. Bryan denied a request by Fors for a temporary restraining order that would have delayed a hearing by a special military panel that is considering changing the status of his son, a pilot who was 26 when he was shot down in December 1967.

In a telephone interview yesterday, Gary Fors' mother, Bobbie, said she and her husband have spent more than $15,000 in attempts to locate their son. She said their belief that he is alive is based on a 1973 North Vietnamese propaganda film that shows a man they believe is their son.

"We're not so naive as to think that something hasn't happened to him, but that photograph keeps haunting us," she said. "There's no proof that Gary is dead."

She said she believes that a declaration of death would halt government efforts to find her son, one of 79 U. S. servicemen officially listed as missing in action.

Defense Department officials deny that an official declaration would halt their efforts to locate missing Americans like Gary Fors.

"Each of these cases is thoroughly reviewed," said Pentagon spokesman Samuel W. Floca, "and a reclassification in no way affects our attempt to find these men. We still remain committed to as full an accounting as possible.

"But we have no credible evidence that would indicate that American servicemen are in captivity and being held against their will in Southeast Asia," Floca said.