There, initially I asked him to position the pedal so that he could start pedalling off. After some attempts, this didn't seem to work, partly because the chain kept dropping off with this sudden assert of force on the pedal. Ironically, another neighbourhood kid of his age, who hasn't yet able to ride a bicycle, suggested to push first then step on. So, he tried that and after some attempt, he could pedal and balance for more than 2 m. Upon see this, I concluded he has learned how to ride.
I think the short distance was mainly due to the chain was very loose and kept dropping off. Later, than I realized the problem was because when I fixed the rear wheel earlier, I didn't ensure the chain is taut, which is not necessarily when there is a rear derailleur, but this is a single speed bike, the first that I have ever meddle with. Now that this issue is resolved, I am eager to see him ridding a bicycle.
Update: Confirmed he can. The night this blog post was published, he showed his ability to ride a bicycle to my dad.