Teaching a child to push off
In my experience, kids don't realise that they have to push hard at the start. 

It seems obvious (even "intuitive") to us, but not to them.

On the balance bike, one can just push along gently, but this new bike is bigger and heavier, and the gearing makes it harder.

So my advice is to encourage him with an enthusiastic "push hard" call. Or go further and tell him to stand on the pedal.

I solved this problem in a different way: I built my son's bike with a gear cluster at the back (no derailleur) with the chain on the easiest gear. As he got stronger, I gradually moved the chain across one cog at a time.

The main thing is to have fun with them.