(CNN) -- Too weak to stand or walk, William M. LaFever sat in a shallow river bed in the south Utah desert, awaiting rescue that came more than a month after his family last heard from him.
Thursday afternoon, a helicopter flying in the Escalante River gorge spied the bearded LaFever, 28, who had lost 50 pounds and eaten frogs and roots in his desperate effort to stay alive during his walk from a Utah city to Page, Arizona.
"We came around the corner and we were pretty amazed to see him alive and sitting up," Shane Oldfield, a Utah Highway Patrol helicopter pilot assisting the Garfield County Sheriff's Office, told CNN Friday.
LaFever was hospitalized Friday in St. George, Utah, in stable condition. Family members told a deputy that he has autism and obsessive-compulsive disorder, said Becki Bronson, a spokeswoman for the sheriff's office.
The Colorado Springs, Colorado, man's incredible odyssey began when he called his father on June 6 or 7 to tell him he was in Boulder, Utah, hiking with his dog, according to the sheriff's office.
LaFever said he had run out of money and someone had stolen some of his hiking gear. Authorities said they assumed he was given a ride to Boulder, as he did not have his own vehicle.
"He didn't want me to come out there," said LaFever's father, John LaFever. "He wanted me to send him some money to get him to Page."
The elder LaFever said he wasn't sure how William would make it all the way to Page, but his son mentioned following the river.
Within a week of his call home, William got a ride to where Highway 12 crosses the river, and set out on his journey.
The younger LaFever apparently did not know the extent of the rigors he would face when he planned to walk nearly 50 miles in the Escalante Desert to Lake Powell and then obtain a boat ride, officials said.
Eventually, LaFever's food and strength ran out as he continued walking, according to authorities, and his dog ran off. Officials were looking into a report that the animal may have been located, Bronson told CNN.
With her brother long overdue, LaFever's sister called authorities Monday, seeking help and providing information on where he might be.
Garfield County Deputy Ray Gardner accompanied Oldfield on the flight and told him he had learned in recent training that people with autism often are drawn to water.
Oldfield said the hiker had apparently followed the river as he walked south and had been in the location where they found him for several days, sapped of almost all his strength. LaFever was only a few miles away from Lake Powell.
"I think he probably hiked as far as he could until he was physically exhausted and he went into survival mode," the pilot said.
LaFever had no compass and had discarded or lost his hiking gear.
All he had were his pants, shoes, underwear, shirt and a lighter.
He used the lighter to set a small fire at night, officials said, and then would roll into the river to stay cool and drink water.
LaFever weakly waved when Gardner and Oldfield spotted him from the air.
After their landing, the pair approached the emaciated man, not certain of his identity. Initially, LaFever did not want to get into the helicopter.
"He was asking us for food but it was not like he was shoveling it down. He was casually eating a granola bar as he was talking to the deputy," said Oldfield.
In a statement, Gardner said he had not been expecting a good outcome.
"We had no idea if William had stayed along the river, or decided to leave, or got a ride with someone, or was lost somewhere other than along the river," he said. "We flew along it without any expectation of finding anything at all. The conditions would have to be just right too; unless he was out in the open there was no way we would be able to find him."
Gardner, praised by LaFever's family for his work on the case, did not immediately return a message left Friday by CNN.
William LaFever was in an area popular with hikers, but was about 30 miles beyond where most venture, Oldfield said.
"He made the mistake 'I know what I am doing and I will be OK,''' Bronson told CNN. "There are many who have done that and paid the price."
John LaFever said his son "has done a lot of camping but he has always been close enough to where he could walk to people and get a grocery store. It has never been at this capacity."
William LaFever, who is on disability, will be hospitalized indefinitely as he regains his strength.
"I am just overwhelmed," said his father. "I was really hoping he was alive. The chances of him being found alive were one in a million, maybe."