The Kazakhstan native is charged with obstruction of justice. If found guilty, he could face up to five years in prison and $250,000 in fines.
He is being represented by Arkady Bukh, an attorney based in New York.
Ismagulov said his son always admired and wanted to come to the United States. He was here to study engineering and work in the oil business, his father said.
Teenagers sometimes do stupid things, said Ismagulov, stressing that his son didn't know he was doing anything wrong.
Ismagulov said that he asked his son whether he had wanted to help Tsarnaev.
He apparently told his father no, saying that if they had wanted to help Tsarnaev, he and Kadyrbayev would have thrown out the bombing suspect's laptop and buried his backpack in the ground.
Investigators found the backpack, loaded with fireworks, in a landfill after a two-day search.
Tsarnaev's laptop was turned over by Kadyrbayev on April 19, the same day the FBI raided the apartment he shared with Tazhayakov, Kadyrbayev's attorney Robert Stahl said.
According to the University of Massachusetts-Dartmouth, Tazhayakov is enrolled but has been suspended pending the outcome of the case.
Kadyrbayev also remains in jail, awaiting a May 14 court date.
According to an FBI affidavit, Kadyrbayev had seen pictures of the suspects released by the FBI on April 18 and texted Dzhokhar Tsarnaev to tell him "he looked like the suspect on television." Tsarnaev texted back "lol" and added, "come to my room and take whatever you want."'
Attorney Stahl also said his client "did not have anything to do" with the bombings and disputed that he tried to block the investigation.
Kadyrbayev, a Kazakh national, was taken into custody along with Tazhayakov on April 20 on suspicion that he had violated the terms of his student visa, Stahl said.
According to an interview his father gave in April, Kadyrbayev, 19, "missed a couple, or maybe several classes."
"I can say about my son that he finished school with excellent grades; he was good at math. He helped others. When he saw that help was needed, he always accommodated," Murat Kadyrbayev told Tengi News and STV channel in Kazakhstan.
Kadyrbayev is not currently enrolled at the University of Massachusetts-Dartmouth.
He is charged with obstruction of justice and could face up to five years in prison and $250,000 in fines if found guilty.
Kazakhstan's foreign ministry said it was offering consular services to both Kadyrbayev and Tazhayakov. "We would like to emphasize that our citizens did not receive charges of involvement in the organization of the Boston Marathon bombings. They were charged with destroying evidence," the ministry said in a statement.