A six-year-old autistic boy missing since early Saturday morning has been reunited with his family after turning up in a homeless encampment four miles from his home in California.

Sergio Zepeda disappeared from the home he shares with his mother Augustina Quinteros and her boyfriend Bony Sanchez around 9am on Saturday.

He turned up, naked and shivering, outside the tent of homeless man Jose Salmeron, 32, around 4am Sunday morning.

Safe and sound: Six-year-old Sergio Zepeda turned up miles from home in a homeless encampment

Quinteros told the Mercury News that her son was watching cartoons on Saturday at their home on the 2,900 block of Rorty Drive in South San Jose and jumping on the couch.

She and Sanchez noticed he had gone very quiet and upon investigation found that the garage door was open and the boy was missing.

After spending an hour searching for the boy, who Quinteros says has wandered off before but has always been found or returned by himself, the couple called 911.

The little boy is autistic and cannot communicate in full sentences, making the search for the boy even more difficult.

Police had been searching for little Sergio for hours when Jose salmeron was awoken in the early hours of the morning by scuffling sounds outside his tent.

'I popped out of my tent and I saw a naked person. At first I thought it was an alien, like ET,' Salmeron told the Mercury News.

Far from home: Sergio was found four miles from his home, which is on the opposite side of this hill

Stark naked: Jose Salmeron was shocked to find a shivering, naked little boy outside his tent at 4am

Long walk: Little Sergio wandered almost four miles from his home, but police believes he walked over a hill, avoiding main roads

'I shined my flashlight at him and when I looked closer it was a little boy.' He yelled to others in the encampment, 'Come out, there is a naked boy out here!'

When he left home, Sergio was wearing jeans and a Super Mario Brothers t-shirt, but his mother said he was likely to have stripped them off.

His feet were dirty and he appeared cold, hungry and thirsty.

Salmeron wrapped Sergio in a blanket and gave him some food.

'He wouldn't say anything, but I asked him if he wanted "agua," in Spanish, and he said "agua." He was really thirsty,' Salmeron said.

Frantic: Augustina Quinteros (left) and her boyfriend Bony Sanchez (right) realized Sergio (back) was missing on Saturday morning

Relief: Sergio's mom Augustina Quinteros and her boyfriend Bony Sanchez were reunited with him at hospital

Meanwhile, helicopters and search and rescue teams combed the area around Sergio's home.

One of the people living in the encampment called 911 and the police, after some difficulty locating the encampment, came to pick Sergio up and reunite him with his family.

'It's a happy ending,' San Jose police officer and spokesman Albert Morales told The Mercury. 'We're tremendously fortunate that he was safely found. Here's a six-year-old out in the elements, and he trekked a pretty good distance. We're all just very happy that it ended the way it did.'

Sergio was taken to hospital to be examined and was given a clean bill of health by doctors.

According to KTVU, police want to nominate Salmeron for a Good Samaritan award, but Salmeron doesn't feel deserving of praise.