Making the pedals on my kid's bike rotate when I push so he'll learn to pedal
So your boy is simply sitting and not pedalling, so you push him along?  Of course he's not going to pedal when he's getting a free ride from you.

Stop Pushing!

Firstly, does he understand how a bike works?  Do you ride?  Does he see you or others riding?

You might consider going the other way, and remove the whole transmission and the training wheels.  Essentially revert the bike to a balance bike until he gets the idea that he has to push.

Something like this:


Eventually he'll realise how slow the balance bike is, and want his pedals back to go faster.  And that's awesome.