A relative of Colt's mother said the Gray patriarch felt his 14-year-old was too "tender" and began calling him names -- "sissy" and "bitch" -- to underscore his disdain ... this according to the NY Post.

The relative claims Colin bought his son an AR-15 style rifle for XMAS to "toughen him up." The rifle is the weapon Colt allegedly used when he shot up his high school, leaving 4 dead and 9 injured.

As for the verbal mistreatment, the relative believes Colin was trying to "break him down and emasculate him."

She added ... "That whole mentality of toughening him up and making him stronger. That was Colin’s top priority, and you see where that got us."

Charles Polhamus, Colt's grandfather, expressed a similar sentiment ... painting Colin as an "evil" and abusive figure in the teenager's life, saying, "Spending 11 years with that son of a bitch screaming and hollering every day -- it can affect anybody. He was a good kid turned bad in a bad situation."

Colt has been charged with 4 counts of murder and is being tried as an adult for the shooting, prosecutors confirmed last week. If convicted, he could be sentenced to life in prison.

As for Colin, he has been charged with 4 counts of involuntary manslaughter, 2 counts of second-degree murder, and 8 counts of cruelty to children in the second degree for allowing his son to possess a weapon.