Nathaniel Pope moved into Alicia Watson's West Bromwich flat in the spring of 2018, and six weeks later Kemarni died from massive internal injuries which included more than 20 rib fractures.
Pope, 32, was found guilty of murder and child cruelty while 31-year-old Watson was convicted of allowing a child to die and child cruelty.
Kemarni's father Darren Darby addressed the court through a victim impact statement which he is "serving a full life sentence" because of his son's death.
He said: "Kemarni was an active, fun, cheeky, boisterous young boy, he was always smiling and laughing. I would describe him as my little best friend and he has been taken away from me.
"He had the potential to be so much. I will not get to see him grow up. I will not get to share special times with him, and be there for key moments in his life.
"Kemarni was loved by so many friends and family. We are a close-knit family and this has impacted so many people.
"I have nothing but hate for Pope and Alicia. I trusted her to look after my child.
"They will do their sentences and get on with their lives, I am serving a life sentence because Kemarni has gone forever."
Pope, of Evans Road, Wolverhampton, had previous convictions for drug dealing and battering a woman on a bus in London in 2011 in front of her children. When arrested for the murder of Kemarni he was found to have rock of crack between his buttocks, the trial heard how both were frequent cannabis users.
Watson, of Radnor Road, had no previous convictions and was described as "suffering from post natal depression" in the run up to the murder. The couple remained together for almost a year after Kemarni's death.
Justice Amanda Tipples is due to sentence Watson and Pope tomorrow (Tuesday).