Simons left the home he shared with his mother Eliza and Roberts when he was 16 after a drug-fuelled argument with the actor, during which he attacked his stepfather with a baseball bat and broke his ribs.
The two had not spoken since the fight - until Roberts taped the current season of Celebrity Rehab and Simons agreed to come face to face with his stepfather.
Prior to the tense meeting, Simons sat down with Celebrity Rehab host and counsellor Dr. Drew Pinsky to explain what led to the estrangement.
He revealed, "With Eric, it got pretty crazy... We formed a drug bond over marijuana... He introduced me to it in a real way... (and) I was getting drugs for him and he'd get mad at me because I was ripping him off... It started to get dangerous.
"I saw a lot of angry things. He was very unpredictable and I thought very unsafe and one time we got into a fight... With him it was mostly challenges... and insulting me.
"One night I was playing music way too loud for that late hour and he came upstairs and was screaming... I went out and I got a bat... and I hit him in his ribs and broke his ribs and then I punched him and hit him in the head and then I left. He was still taunting me when I left."
And Simons told Dr. Drew he was reluctant about meeting his stepfather, who was trying to break his marijuana habit in rehab: "It's hard for me to be compassionate to him in his condition. I don't really care if he gets better or not."
But the pair quickly made amends and Roberts insists his stepson is now a big part of his life.
The actor explains, "We're great. We speak several times a day every day now. We're back to the same closeness we had before we had difficulties. It's really wonderful.
"He wrote the music for a movie I did called The Novelist and he wrote music for Celebrity Rehab and also for a show called Grace."
Asked about his stint on Celebrity Rehab, Roberts - the brother of actress Julia and father of Emma - states, "Celebrity Rehab was quite an endurance test and it let me know that everything comes a step at a time."