The 27-year-old killed girlfriend Steenkamp by firing four gunshots through a toilet door at his home in South Africa in the early hours of February 14, 2013.
Life imprisonment is not an option for the double amputee, but he could face a maximum term of 15 years behind bars when he is sentenced on October 13.
In an interview with the Mail on Sunday, Adam Steenkamp said: "In my heart I know he has got away with it. He has got away with murder.
"I don't understand the logic of acquitting someone of murder who fires four rounds into a very small toilet cubicle. It is not the action of someone not intending to kill."
When referring to his father Barry and the rest of Reeva's family, Adam said: "You try not to be too angry because that seems the wrong thing to do. We are looking for the positives and what can come out of this and make things better instead of worse."
Pistorius denied murder by maintaining that he mistook his girlfriend for an intruder when he fired the gun.