Majuro, Marshall Islands (CNN) -- After showing signs of improvement, Jose Salvador Alvarenga's health has deteriorated.
Alvarenga, who said he was lost at sea for 13 months, returned to a hospital Thursday to be intravenously fed, said Christian Clay-Mendoza, a top Mexican trade official.
"Doctors say he's severely dehydrated and low on vitamins and minerals," he said.
Alvarenga, an El Salvadoran who had been living in Mexico, washed ashore on the Marshall Islands eight days ago. Earlier this week, his condition improved enough that he was released from a hospital.
Doctors said Alvarenga's limbs have started to swell, and they can't seem to keep him hydrated.
But he did manage to leave the hospital briefly Thursday to make a few comments to the media.
Bewildered by all the cameras, he thanked the government and people of the Marshall Islands for their care and friendship. After his minute-long remark, officials whisked him back to the hospital.
Plans for his repatriation to El Salvador on Friday have now been postponed due to his health.
Official: His story 'is probable'
Alvarenga's claims have garnered widespread skepticism about how he could survive for 13 months adrift on the Pacific. But from what officials can tell, they have no reason to doubt him.
"The investigations into Mr. Alvarenga's story so far have been substantiated," said Phillip Muller, the Marshall Islands' foreign affairs minister.
Clay-Mendoza said Alvarenga was an undocumented worker in Mexico. But "what he has said to us about his identity, so far, has been true."
"We've had contact with his family in El Salvador, and they have corroborated his story," the Mexican official said. "Now we are trying to contact his boss at the fishing co-op he belonged to in Mexico, but so far everything he's told us has jibed. The main question now is how long was he at sea?"
Clay-Mendoza said "it's probable" Alvarenga really did get lost at sea starting in December 2012. He said the boat was reported missing in 2012.
If Alvarenga's story proves true, the trip across the Pacific would have taken him across roughly 6,600 miles (10,800 kilometers) of open ocean before ending in the Marshall Islands, about halfway between Hawaii and Australia, in the northern Pacific.
The other man
Alvarenga says he set off on a fishing trip from the port of Paredon Viejo, Mexico, near the southern coastal city of Tonala.
He said he and another man intended to spend a day trying to catch sharks, but they were blown off course by winds and then got caught in a storm, eventually losing use of their engines.
Bellarmino Rodriguez Beyz, the owner of Alvarenga's boat in Mexico, identified the fishing partner as 23-year-old Ezequiel Córdova.
Alvarenga said that four weeks into their drift, his companion died of starvation because he refused to eat raw birds and turtles. Eventually, he threw the body overboard.
"What else could I do?" Alvarenga said.
A grieving family
Back in the Mexican village of El Fortin, Córdova's family is inconsolable. More than a year after the young man went missing, his mother is grieving his death.
"The pain is so great, I can't explain it," Córdova's mother said. "Losing a child is the hardest thing to bear in life."
Córdova's brother said the 23-year-old took care of his family -- and that's why he became a fisherman, in hopes of earning a better livelihood.
"My brother was kind, he was responsible for my mother," he said. "In fact, he worked in the sea because of her. He wanted to improve himself. He didn't want to be poor, like us."
The fishing cooperative that Córdova and Alvarenga belonged to has about 45 boats, Rodriguez said. The fishermen make about $150 a week.
But the waters near Paredon Viejo are notorious for the dangerous winds that can blow fishermen away.
Now that Córdova's mother knows her son is dead, she wants answers.
"As a mother, I demand the authorities allow me to talk to the survivor," she said. "Only in that way will I know what happened, and what he did with the body of my son. I deserve to know the truth. "
Meanwhile, Córdova's bedroom, scant with just a blanket as a headboard, remains untouched.
Just the way he left it more than a year ago.
Suzanne Chutaro and Jack Niedenthal reported from Majuro, Marshall Islands; Holly Yan reported from Atlanta. CNN's Nick Parker and Jethro Mullen also contributed to this report.