Dylan Redwine, 13, mysteriously vanished in November 2012 during a visit to see his father in Colorado, USA.
The boy’s remains were discovered scattered in the mountains of South-western Colorado seven months later.
Mark Redwine was arrested last week in Bellingham, Washington, in connection with the death.
And Dylan’s brother Cory has now come forward to say he and his sibling saw lewd photos of their dad carrying out a series of bizarre acts before his death, ABC7 reports.
The brothers had been planning to confront their father over what they had seen, according to the site.
Cory is quoted as saying: “I have seen the photos. It shows him wearing women’s clothes and makeup and a diaper and then eating his faeces from the diaper.
The boys’ mother Elaine Hall said on a podcast in 2015 that she believed her son might have said something to her ex which sent him into a rage.
She said: “He may have said something that just didn’t set well with Mark and Mark reacted in a very violent way…obviously. Dylan is no longer here.”
The parents had gone through a contentious divorce and custody battle before their son’s death, which Mark Redwine has always denied any involvement in.
The couple hurled accusations at each other during appearances on the Dr Phil TV show in 2013 as the case drew national attention.
After his disappearance, some of Dylan’s remains were found around 10 miles from his father’s home in Vallecito in 2013.
The boy’s skull was found by hikers in 2015, and forensic experts said it showed injuries consistent with blunt force trauma in two places, the indictment said.
The boy had reluctantly travelled to be with his father for a court-ordered visit the previous Thanksgiving.
Surveillance video from the airport where Dylan’s flight arrived shows little or no interaction between the boy and his father, according to the indictment.
Several witnesses said Dylan did not want to visit his father after the two argued and fought during his previous visit.
Text messages showed that Dylan had asked to stay with a friend on the night of his arrival, but Mark Redwine denied the request, the indictment said.
The next morning the boy’s friend sent a text to Dylan asking “where are you” and got no response.