SLEEPY EYE, Minnesota (CNN) -- A 13-year-old Minnesota boy whose family has rejected chemotherapy to treat his cancer is with his mother near Los Angeles, California, and they may be planning to travel to Mexico, authorities said Wednesday.

Doctors say Daniel Hauser's lymphoma responded well to a first round of chemotherapy in February.

Brown County Sheriff Rich Offmann cited "reliable information" in making the announcement to reporters, adding that Colleen Hauser may be seeking treatment for her son's lymphoma just south of San Diego, California, in Mexico.

"I'm confident we will find them," Offmann said. "I'm hoping for Daniel's sake we will find them."

Anthony Hauser, Colleen's husband and the boy's father, has been cooperating with law enforcement, Offmann said.

Earlier Wednesday the boy's father said he believed his son and his wife had already left the country.

"I will say this: I have left a call to where I think they could possibly be," Anthony Hauser said from his home in Sleepy Eye, Minnesota, about 85 miles southwest of Minneapolis.

Asked whether he believes that they have gone to Canada, he said, "I'm not saying it's Canada. You know, that isn't where I left my call."

He made his comments a day after a Minnesota judge issued an arrest warrant for Colleen Hauser, who failed to appear with the boy at a court hearing. A judge scheduled the hearing to review an X-ray ordered by the court to assess whether Daniel Hauser's Hodgkin's lymphoma was worsening.

A doctor testified at the hearing that the X-ray showed "significant worsening."

In a news release, the Brown County Sheriff's Office said the father had been "cooperative" in helping them find his son but added, "the investigators cannot speculate on the sincerity of the information that Anthony Hauser has provided."

The sheriff's office has been in touch with the FBI, and the boy has been entered in the Missing and Exploited Children network, it said.

Anthony Hauser testified at the hearing that he last saw his wife at the family's farm on Monday night, when she told him she was going to leave "for a time."

He later said he would like his wife and son to return.

"I'd like to tell them, you know, 'Come back and be safe and be a family again,' " he said. "That's what I'd like to tell them." Watch father urge the pair to come back »

District Judge John R. Rodenberg of Brown County, Minnesota, said the boy's "best interests" require him to receive medical care. His family opposes the proposed course of treatment, which includes chemotherapy.

"It is imperative that Daniel receive the attention of an oncologist as soon as possible," the judge wrote.

During Tuesday's hearing, Dr. James Joyce testified that he had seen the boy and his mother on Monday at his office. He said the boy had "an enlarged lymph node" near his right clavicle and that the X-ray showed "significant worsening" of a mass in his chest.

In addition, the boy complained of "extreme pain" at the site where a port had been inserted to deliver an initial round of chemotherapy. The pain was "most likely caused by the tumor or mass pressing on the port," testified Joyce, who called the X-ray "fairly dramatic" evidence that the cancer was worsening. Watch CNN's Dr. Gupta discuss Daniel's chances »

Rodenberg ordered custody of the boy transferred to Brown County Family Services and issued a contempt order for the mother.

Philip Elbert, Daniel's court-appointed attorney, said he considers his client to have a "diminished capacity" for reasons of his age and the illness and that he thinks Daniel should be treated by a cancer specialist.

Elbert added that he does not think Daniel -- who, according to court papers, cannot read -- has enough information to make an informed decision regarding his treatment.

Daniel's symptoms of persistent cough, fatigue and swollen lymph nodes were diagnosed in January as Hodgkin's lymphoma. In February, the cancer responded well to an initial round of chemotherapy, but the treatment's side effects concerned the boy's parents, who then opted not to pursue further chemo and instead sought out other medical opinions.

Court documents show that doctors estimated the boy's chance of five-year remission with more chemotherapy and possibly radiation at 80 percent to 95 percent.

But the family opted for a holistic medical treatment based upon Native American healing practices called Nemenhah and rejected further treatment.

In a written statement issued last week, an attorney for the parents said they "believe that the injection of chemotherapy into Danny Hauser amounts to an assault upon his body, and torture when it occurs over a long period of time."

Medical ethicists say parents generally have a legal right to make decisions for their children, but there is a limit.

"You have a right, but not an open-ended right," Arthur Caplan, director of the center for bioethics at the University of Pennsylvania, said last week. "You can't compromise the life of your child."

CNN's Chris Welch contributed to this report.

All About Cancer • Minnesota