A senior leader of the Cambodian noncommunist resistance said after a top-level State Department meeting yesterday that he is "very optimistic" that the United States will supply military aid to his fighters, but that no definite promises have been made.

Former Cambodian prime minister Son Sann, who is president of the noncommunist Khmer People's National Liberation Front, made the statement after a meeting with Secretary of State George P. Shultz.

"We are discussing the procedure to know what we need and what is good for us," said Son Sann.

He added, "You can't have 100 percent of what you want. If you have 60 or 70 percent of what you wish, that's a good thing . . . . I am very optimistic."

The House Foreign Affairs Committee authorized $5 million in military aid last week to the noncommunist forces fighting Vietnamese troops in Cambodia, despite the Reagan administration's opposition to direct U.S. military aid.

Since the House action, official statements have left open the possibility that the administration might change its position if the military aid is approved by both houses of Congress. A State Department statement yesterday, before the meeting of Shultz and Son Sann, said the administration is convinced that the resistance forces do not need U.S. weapons now, "but we do not think it is wise to forgo having flexibility on this point should circumstances change."

State Department sources said following the meeting that Son Sann was told the administration would be willing to use the $5 million to supply nonlethal support such as food and economic assistance. However, the official said the administration's view remains that the noncommunist resistance has sufficient weapons from other sources to arm its 25,000 fighters.

Son Sann was accompanied to the State Department meeting by Prince Norodom Ranariddh, son of the former Cambodian head of state, Prince Norodom Sihanouk. The younger prince is commander of the Sihanouk-sponsored element of the Cambodian resistance.

The communist Khmer Rouge ousted the pro-U.S. government in Cambodia 10 years ago next week, shortly before Saigon fell to the Vietnamese communists. The Khmer Rouge regime was responsible for the death of up to 2 million Cambodians before being ousted from power by a Vietnamese invasion beginning late in 1978.

The Khmer Rouge headed by Pol Pot is the most militarily important element of the current Cambodian resistance and strongly backed by China. The noncommunist elements headed by Son Sann and Sihanouk are in a coalition with the Khmer Rouge against the Vietnamese, but are seeking their own weapons from the West to sustain an independent base of power.

After his meeting, Son Sann said that noncommunist Asian nations as well as China have provided help for his rebel forces and that both encouraged him to seek further aid from the United States.

In a brief interview, Son Sann said the start of U.S. military assistance even on a small scale would be of great value. "We believe that the most important thing is opening the tap. I hope that afterward the flow will increase and be sufficient."