Why is my son not interested in his studies?

My son is 6 years old and is in the 1st standard. He is not interested in his studies. He wants to play all the time. The problem is that he is very slow in writing. I think he is not concentrating in the class. He is the last person to finish in the class, while everybody else finishes. I am motivating him in many ways and speak to him politely too. But he just doesn't seem to understand.