The end result looks great and he did a good job but what I wonder about is why you never taught him at a young age how to draw and paint. My children held a pencil at the age of not even one year old and at the age of 2 I gave them a huge wall in the hallway to draw and paint (making a fresco is cool too). Two hours is indeed not much and I hope you will not restrict his love for art by telling him he can only paint if he studies hard. To be honest it sounds to me that by saying this you also have no faith in the fact one can make a living with art. That saddens me and I made sure my children can and they do even if they need a part time job to pay for what they love to do.

By the way instead of a finger a brush but also a sponge can be used, It has a great effect and goes way faster. We also use a sponge to paint with if it's for bigger surfaces. I am looking forward to your next paintings! A big compliment to your son.