Joseph Bell, 48, started his walk from Oregon to New York in April. His 15-year-old son, Jadin Bell, took his own life earlier this year after relentless bullying at school and social media over his sexuality, according to the Joe's Walk for Change Facebook page.
"Joseph Bell is walking across America to educate anyone who will listen about the lasting effects of bullying," the site said.
Bell began his walk in April, just months after his son died.
"He was hurting so bad," Joseph Bell told ABC affiliate KATU. "Just the bullying at school. Yeah, there were other issues, but ultimately it was all due to the bullying, for not being accepted for being gay."
Jadin Bell attempted to hang himself in January at an elementary school playground in La Grande, Ore., about 250 miles east of Portland, Ore. He was taken off of life support and died on Feb. 3.
Wednesday's incident happened along a rural stretch of U.S. 40, about 20 miles northwest of the town of Kit Carson, the Colorado State Patrol said. Bell was walking along the shoulder of the road at about 5:10 p.m. when he was hit. He was pronounced dead at the scene.
The driver of the big rig was identified as Kenneth Raven, 49, of Bryan, Texas. Colorado State Patrol Trooper Josh Lewis tells ABC News that investigators believe Raven fell asleep while he was driving for a company called Farold Inc. Raven was not injured in the crash, Lewis said, but was cited with careless driving resulting in death.
Reached by phone Thursday, the owner of the trucking company expressed sympathy over Bell's death, but says Raven disputes the police account.
"He says he did not fall asleep," said Greg Cupit, owner of Farold Inc.
Cupid said he could not comment further about the incident, saying he needed to learn more about what happened.
On Facebook, hundreds of people posted messages of support.
"Thank you so much for all of your continued support, he loved everyone he came into contact with, and was so appreciative of all of your support," read one Facebook post. "He will continue his journey now with Jadin. Please keep his family in your prayers and thoughts."
On Oct. 8, Bell posted a video of himself walking on Highway 40 leaving Limon, Colo., headed for Wichita, Kan. Traffic could be heard passing nearby. Bell told the camera he expected to make it to Wichita in about five weeks.
"I'm no spring chicken anymore, that's for sure," he joked in the video. "But this is what I'm out here for, to make change."
Bell left behind his wife, Lola, daughter, Amber Lynn, and two sons, Dustin and Joseph Jr.
New York was Bell's destination because that is where Jadin eventually wanted to live because he felt he would be accepted there, according to Bell's website.
A photo of a road sign posted on the page Wednesday before Bell was killed indicated he was about 20 miles outside Kit Carson.
"Making headway, one step at a time," the caption read.