Police were looking Wednesday night for a 19-year-old man charged in connection with the deadly brawl.
Kevin Kennelly, Jr., 17, was a student at Mount Carmel High School on Chicago's South Side.
His father says Kennelly was trying to stop the fight that happened on the night of the Fourth of July.
The fight happened in Long Beach, Indiana, just northeast of Michigan City.
Police announced aggravated battery, battery, and involuntary manslaughter charges against Chicago resident James Kieffer Malecek.
Ribbons adorn the neighborhood where Kennelly grew up in a tribute to a young man who was a joy to his family.
"He was a peacemaker, and he was a kid who didn't have any enemies," said Kevin Kennelly, Sr. "He was my only child, and from the day he was born, he was never any trouble to me."
Kennelly, Jr. was with friends along the lake in the idyllic waterfront community of Long Beach, Indiana.
His father, a Chicago Fire Department captain, says a fight broke out on the beach and his son stepped in to break it up.
"Somehow or another, as he was doing that, the offender sucker-punched him in the face, and he went immediately down," said Kennelly, Sr.
Kennelly, Jr. was rushed to St. Anthony's Medical Center with bleeding in his brain, according to his father.
He died Tuesday morning, and the cause of death yet to be determined. An autopsy is scheduled for Thursday.
"I told this kid from the time he was small: don't try and get in the middle of a fight and stop it, but he didn't listen to that," said Kennelly, Sr. "He listened to his 13 years of Catholic education that said, 'Blessed are the peacemakers.'"
Kennelly, Jr. was to be a senior in the fall at Mount Carmel, where multiple generations of his family attended school.
On Tuesday, Mount Carmel had a mass in honor of the two-sport athlete who played this past spring for the state semifinalist baseball team.
"All friends of his, all kids who might just have had him in one class, and they just thought what a nice kid. He was just a lot of fun. He didn't have any enemies," said Kennelly, Sr.
About 500 people attended the prayer service, which was described as a beautiful ceremony. The Mount Carmel lacrosse team posted a request for prayers on its Facebook page.