Lachey, who as been a "Big Brother" since 2004, is teaming up with the program to raise funds during the second annual "World's Largest Waterslide-athon" at the Great Wolf Lodge.
Lachey talked to Parade.com about his years in the program and how being a "Big Brother" has prepared him for fatherhood.
On his role as a "Big Brother."
"I've been with my Little Brother for seven years. He's 15 now, and the evolution of seeing him go from a shy, timid little boy to a young man in high school who has completely come out of his shell has been pretty awesome to be a part of. If I've affected his life in even the smallest of ways, it's been more than worthwhile."
Playing big brother to his real-life bro Drew was more of a challenge.
"Taking care of Drew was heads and tails harder than anything I've had to deal with in Big Brothers Big Sisters. He was a pain!"
He's looking forward to his next role: fatherhood.
"I'm assuming fatherhood is going to be a hundred times more intense than being a Big Brother, but watching a life develop in front of you is a bit of a parallel. I'm certainly looking towards it. I'd like to be a dad sooner than later. I'm getting up there. I've gotta catch up with my brother. He's got two and I've got zero, so the pressure is on me!"
Nick and Vanessa went head-to-head in a dance-off at their recent wedding shower.
"Well, I wouldn't call it a dance-off. I attempted to dance and she humored me. There's no competition when it comes to my dancing abilities. I didn't get that gene in the family. Drew got that one."
What's in store for his upcoming bachelor party?
"I'm fearful! But we're going to have a great time. At this point in life you don't get to see the boys as much as you used to and a lot of the friends from college and stuff have moved on and have their own families. This is a chance for us all to get together and tie one on and have some fun again. We're all looking forward to it."
Why he can't wait to say "I do."
"I'm at a point in my life where I'm ready to start another chapter. Having kids is a big part of that. I'm excited for what comes next. Life is a journey, and I'm certainly ready to close this chapter and move on to the next."
Can his bride-to-be expect a serenade?
"If she acts right, she might get a serenade! That's my trump card. Whenever I screw up and I need to get back to her good graces, I sing. So far it's worked."
The key to having a "normal" relationship despite the fame.
"I think the secret is just to ignore all that stuff. People are always going to write and say what they want to. What matters at the end of the day is what happens between two people, and that's what we try to focus on. All the other stuff is out of our control."
Can fans expect a 98 Degrees reunion a la Backstreet Boys and New Kids on the Block?
"We've left the door wide open on that one. There's certainly no animosity between any of the members. We're all friends. If their schedules permit and the opportunity is there, I certainly see it happening down the line."