"I miss him all the time," Williams's oldest son, Zak, 31, tells PEOPLE in this week's cover story.
"Often I see something or if I'm watching a film, I think, 'Oh, man, he would have appreciated this' or 'He would have gotten a laugh out of this.' "
Speaking for himself and siblings Zelda, 25, and Cody, 23, he adds, "There's not a day that goes by that we don't think about our dad."
Zak, whose mom is Williams's first wife, Valerie Velardi, is one of several loved ones who spoke to PEOPLE about the late Oscar winner's lasting impact and his legacy of compassion and generosity.
Marlo Thomas, who worked with the late star for 18 years through St. Jude Children's Research Hospital, recalls his extraordinary connection with young patients.
"One of them liked toy soldiers, so Robin sent him his own toy soldiers that he had saved all those years of his childhood," says Thomas. "When he came there, it was like magic."
Honoring his father's spirit of giving back has been comforting, says Zak, who volunteers at a local prison.
"It's through helping others that I've begun to heal," he says.
Richard Corkery / NY Daily News Archive / Getty
Reporting by SHARON COTLIAR and JULIE JORDAN
For much more on Robin Williams's legacy – along with remembrances from the cast of one of his last films, Night at the Museum: Secret of the Tomb – pick up the new issue of PEOPLE, on newsstands Friday – or read it now on PEOPLE Premium