It's manslaughter because there was no planning involved. He killed him in a fit of spiteful and malicious rage. He thought he would hurt him but not kill him. IQ of 0.
To be honest I find his mother to be more culpable than he is. It wasn't his child, and although we're meant to be better than animals, he's a brute and essentially became like a male lion, get rid of the child that's not yours. It was up to her to show him that her child's life was precious.
I think she should get a higher sentence than him. At the end of the day her child turned to her for help and apparently after the first 'crush attempt' she slapped him for complaining. Then he died after the 2nd attempt.
She should be charged with murder. She repeatedly placed her son in danger and played roulette with his life.