If you think it was on purpose or a stiff arm, try video taping yourself running.
I bet your arms move back and forth and that's exactly what happened.
He was running, sidestepped the ref and the boy was pushed be the natural swinging motion of his arm. He didn't look at the boy and change the swing of his arm to push him, it's very obvious from the tape.
What a joke that anyone would think any differently.