Shepard died at his home last week in Kentucky after complications from amyotrophic lateral sclerosis, also known as Lou Gehrig's disease, Boneau said.
"The family requests privacy at this difficult time," Boneau added. Funeral arrangements will be private.
Boneau said Shepard was open with those who asked him about ALS and was not afraid to speak about it.
"He faced the disease head on without an ounce of self-pity," he said.
Shepard, whose work spans more than five decades, was nominated for an Oscar in 1983 for his role in "The Right Stuff." He also had a very successful career off-screen.
He won a Pulitzer Prize for drama in 1979 for his play "Buried Child," one of 40 he wrote over the years.
An Illinois native, he began his acting career in the early 1960s working off-Broadway, then moving to Broadway and the big screen in the 1970s and 1980s. Two other notable plays by him are 1980's "True West" and 1983's "Fool for Love."
On the silver screen and TV, Shepard followed up "The Right Stuff" with "Steel Magnolias" in 1989, "Dash and Lilly" in 1999 (for which he earned an Emmy nod) and "Black Hawk Down" in 2001, among a slew of other projects.
More recently, he starred in Netflix's "Bloodline" in 2015 and was in this year's "Never Here," which hit movie theaters in June.
Shepard is survived by his children — Jesse, Hannah and Walker Shepard — and sisters — Sandy and Roxanne Rogers.