The little boy's father, Jeremy Cramer, is charged with deliberate homicide and jailed on $250,000 bail. Police say he drove his son, Broderick Cramer, from Lacey, Wash., to Montana, then killed him.
The boy's father was arrested after he was seen washing blood off his clothing at a convenience store. After the father's car was located, the boy's body was found lying nearby.
Two local pastors officiated at Thursday's ceremony organized by Aimee Nichols and Rebecca Gallagher of Anaconda.
Anaconda resident Melissa Penner said she was there because she wanted to support Broderick's mother. Penner's son, Tyler Kelly, and Broderick were born just days apart.
Children brought stuffed animals and toys that will be sent to Broderick's mother. Nichols says flowers donated for the service will be placed at the spot where the boy's body was found.
US Bank has set up a "Natalie and Brody Cramer Fund" to accept donations.