Charles S. Whitehouse, former ambassador to Thailand, is expected to be nominated by President Reagan soon to the new post of assistant secretary of defense in charge of the Army's Green Berets, Navy Seals and other special forces, administration sources said yesterday.

The imminent nomination of the career diplomat has helped bring an uneasy truce between the Pentagon and Congress over the long battle to restructure the military bureaucracy to give the special forces more policy direction, money and clout. Key lawmakers in that fight said that Defense Secretary Frank C. Carlucci has been calling them in hopes of winning congressional approval of Whitehouse, his own choice.

Whitehouse, who as president of the American Foreign Service Association in 1982 lambasted the Reagan administration for appointing more political friends to diplomatic posts than "any administration since that of Herbert Hoover," got the inside track for the special forces job after Congress refused to schedule a confirmation hearing for Reagan's first nominee, Kenneth P. Bergquist, a Justice Department assistant attorney general.

Rep. John R. Kasich (R-Ohio), a member of the House Armed Services Committee who helped push the special forces legislation last year, said yesterday that he and other lawmakers will hold off putting any new pressure on the Pentagon to see if Carlucci makes good on his promise to implement the reforms.

Kasich said one question still to be answered is whether Gen. James J. Lindsay, head of the new U.S. Special Operations Command at Tampa, Fla., will draft a five-year plan for the special forces and receive the funds needed to carry it out. Kasich and other champions of more stature for the special forces want Lindsay to have a direct line to the top of the Pentagon when it comes to making budget requests. They contend that Army, Navy and Air Force leaders have treated their special forces units like stepchildren in the past when it came to apportioning funds.

Richard L. Armitage, the Pentagon's assistant secretary for international security affairs, told a special forces panel of the House Armed Services Committee that no longer would the Defense Department fight the reforms that the late representative Dan Daniel (D-Va.), Kasich, Sen. William S. Cohen (R-Maine) and their allies demanded.

Declaring he had "led the charge for the administration against the {special forces} legislation," Armitage said, "We fought a damn good battle and we lost it." He promised that the reforms Congress put into law will be implemented as quickly as possible.

John M. Collins, a military specialist at the Congressional Research Service, told the panel that Lindsay "is in a rowboat with one of his oars missing" because the staff he inherited from the former U.S. Readiness Command at MacDill Air Force Base in Tampa, Fla., has little experience in special operations.

"Improvements" in tailoring the special forces command established last June "have been glacially slow," Collins said.