Dr. Julius B. Richmond, one of Harvard's leading medical professors and a founder of Project Head Start, will be the new assistant secretary for health, informed sources said yesterday.

He will be nominated for a job that until a week ago Wednesday was slated to be filled by Dr. Christopher Fordham III, University of North Carolina medical dean.

Fordham returned briefly to his desk after a session with Health, Education and Welfare Secretary Joseph A. Califano Jr. that morning, canceled all appointments and returned to North Carolina. He wrote Califano a brief, letter, withdrawing his name "for deep personal reasons."

Some HEW officials said he had been dismayed to learn that the job had been reduced in scope by several Califano decisions and actions, including the naming of some of his main deputies.

Officially, Fordham had never been named more than an HEW consultant while he spent a month awaiting official clearances and nomination. Before Fordham agreed to take the job in mid-March, it is known to have been turned down by Dr. David Hamburg, head of the Institute of Medicine at the National Academy of Sciences, and Dr. Charles Sandler, director of Massachusetts General Hospital.

Both Hamburg and Sandler cited personal reasons for the turndown, though associates said they too were concerned with what seemed to be the job's more limited scope.

Several HEW and other sources also said the job was turned down by Dr. William Roy, former Democratic representative from Kansas. One HEW source said yesterday Roy was not offered it.

Richmond, 60, heads the department of preventive and social medicine in both the Harvard medical school and Harvard school of public health. He also heads Boston's noted Judge Baker Guidance Center for disturbed and delinquent juveniles, and was named to President Carter's Commission on Mental Health.

A Chicago native, and University of Illinois medical graduate, he became a pediatrician and a specialist in childhood's mental and emotional problems. From 1965 to 1971 he was medical dean at the State University of New York in Syracuse.

But he was also a consultant and official in the new Office of Economic Opportunity in the Kennedy and Johnson years, commuting often to Washington. He was the first director of Project Head Start, the program to help disadvantaged youngsters by starting their education in the pre-kindergarten years.