On the verge of madness, a grieving man attacks his stubborn stepchild, chasing him onto private property, but when the psychotic landowner captures them he must learn to accept and work together with the boy to escape before they are both killed.

I assume the grieving man's madness is due to the fact he has to care for his stepchild?

Here is another version of the logline you may want to consider:

"A quick-tempered man and his stubborn stepchild must set aside their differences to escape when they find themselves being held hostage inside a deranged psychopath's secluded property".

INTENTION: accept and work together with the boy to escape
OBSTACLE: the psychotic landowner

(The INTENTION and OBSTACLE is the main conflict we're chipping it down to.)

When a psychotic landowner captures a feuding father and stepson, they must work together to escape, before they are killed.

Sounds a lot like an idea I had. I had similar problems. I'll find it for you...