My grandson was much the same at the same age. I took him out on their very quiet dead end road and tried. Held the back straps of his overalls to keep him from falling. Almost total failure: could not pick up the pedals, could not get balanced, could not create forward momentum. Ugh. But a start, eh.
Then something happened in his brain overnight. Got the bike out the next morning, threw his leg over and just took off down the driveway. Hasn’t stopped since.