my son is 4.5 years old, he started to learn to play when he was 3 yrs 9 months. But he started to touch and play the piano by himself when he was 1 year old (as we have a piano at home and we always play it to him).
Sometimes he doesn't like to practise the piano when he finds the piece is difficult. I think that's normal as he is still young!!! His piano teacher says as long as he is willing to practise it everyday (few mins is good enough), it's ok.
I think as a parent, you need to give him alot of encouragement!