"It's a personal issue for me and my family," the singer tells PEOPLE of his brother Zac, who was adopted. Now 19, Zac was diagnosed at age 7. "It's something we've all been helping him with for a long time," his big brother adds.
Lachey, 40, is speaking out as he launches the fifth-annual Lindt Gold Bunny Celebrity Auction on Friday to raise funds and awareness for Autism Speaks. (The auction features porcelain gold bunnies signed by Lachey and such other celebs as Blake Shelton, Kelly Clarkson and Harrison Ford.)
Zac, who lives with their mom Cate in Cincinnati, was diagnosed when he began having trouble learning in elementary school.
"He wasn't learning in the same way as the other kids at school," says Lachey, who now lives in New York City with his wife, Vanessa, and their son, Camden, 18 months, for his new hosting VH1 gig, Big Morning Buzz Live. "They originally thought it was ADD. It took a while to narrow it down."
At the time, Lachey was touring the country with his other brother, Drew, and their band 98 Degrees, and no longer living at home. It was stressful for his mom.
"She was looking for answers more than anything else," Lachey tells PEOPLE. "Once she had them, she could learn to deal with it. It was more stress but at the same time, she could now understand how better to help him."
As a teenager, Zac enrolled in the Cincinnati School of the Performing Arts, the same school his elder brothers had attended.
"He studied technical theater and learned sound, which suited his interests," says Lachey. "And he made friends in the department."
Still, he admits, "Social interactions can be a challenge for him. When he's focused on something, it's tough to get him off of it." Zac, he adds, "is highly functioning and very intelligent. A little introverted, but he's a sweet kid."
One thing he finds pleasure in? Video games. "He's big into them," says Lachey with a laugh. "I'm not hip enough to know the popular ones anymore."
Still the two are close. "No matter what, he's my brother and I treat him that way," he tells PEOPLE. "He's very self-sufficient. I just want him to feel like one of the guys. We joke. He's no different to me."
And that's what Lachey wants to focus on.
"Things don't always work in his mind the way they would in my mind," he says. "Even if people view him as quirky at times, he's genuinely a fantastic guy and a great brother."