Organizers Aimee Nichols, of Anaconda, and Rebecca Gallagher, of Deer Lodge, said the service will take place at 8 p.m. Thursday near the duck pond in Anaconda's Washoe Park.
"There's been a lot of support," Nichols told The Montana Standard. "We've gotten a lot of positive feedback for setting something like this up. People want to know how they can help."
Authorities began searching for the boy after his father, Jeremy Cramer, was arrested Monday night in a convenience store bathroom washing blood from his clothes. The boy's body was found Tuesday about 150 to 200 yards away from his father's pickup on a rural road about five miles south of Anaconda.
Anaconda-Deer Lodge County Attorney Ben Krakowka said Broderick likely died of blunt- and sharp-force trauma injuries, and investigators have found a possible weapon. He declined to say what authorities have recovered, and that additional pathology examinations were needed.
Jeremy Cramer is charged with deliberate homicide and is jailed on $250,000 bond.
Broderick's mother, Natalie Cramer, told police that her husband had left with their child on Monday but did not believe the boy was in danger. The couple had recently been arguing about finances.
A relative released a statement on behalf of the family thanking those who have offered their thoughts and prayers to Natalie Cramer.
The statement asks that the public "respect the privacy of Natalie, who is having to deal with so much loss at this time."
The family has started a fund at Key Bank to help pay for funeral expenses and other bills.
Jeremy Cramer is being defended by Sherry Staedler of the Montana Public Defender's Office. She was in Anaconda Thursday and did not return a call for comment.