It's been only four months since the tragic and untimely passing of legendary comedian Robin Williams, and his family is still very much in the grieving process. Williams' oldest son Zak opens up about his battle in a new interview.

"I miss him all the time," Zak tells People magazine. "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.' "

Zak, the comedian's only son with first wife Valerie Velardi, adds that his younger brother, Cody, and sister, Zelda, continue to struggle as well. "There's not a day that goes by that we don't think about our dad."

"The key thing is just to take things a step at a time," Zak explains. "I know it sounds cliche, but really focusing on what's in front of you, focusing on your well-being, and then trying to fill our hearts so we can be there for others is really important."

One way Zak is learning to become whole again? By giving back through his volunteer work at a local prison.

"It's through helping others that I've begun to heal," he said.