In your opening sentence, you say that your son is 6 years old. At the end of query, you have given his date of birth as Jan. 1995, which would make him 10 years old. I am not very clear, which is the right one.
If he is only 6 years old, you do not have to worry about academics. Even if he is 10, the school must be ensuring some coverage of subjects etc. Children learn from every activity and there is no hierarchy of subjects. I think that a six year old, who is good at drawing does not need a Drawing teacher. He needs large sheets of paper, colours, both crayons and paints, materials of different textures, which he can feel and make into collage or small objects. Or plenty of chalk and smooth floors on which he can scribble. In my experience, teachers of drawing may teach one some technique, but they tend to have stereotyped ideas and images. They are likely to hinder the growth of imagination in a young child. Art is not for something, it is for itself. The process of creating a picture for instance, is more important than the product, especially in childhood.
Assuming your son is 10 years old, I would say that a little bit of resistance to adult authority is quite normal. You can take care of it by spending time listening to your son and show him that you are concerned about him and willing to give him your attention and affection. Ask him what he would like to do and gradually let him see that mastery and working hard are satisfying in themselves.
Plan activities with him that you both enjoy and let the child's mother also do the same. Don't focus only on drawing, but let him do it as he likes. He will ask for help himself. Suggest music or cricket or puzzles. If you take care of the child's interests now, you would be investing in his future. Happiness is the best tonic for mental and physical growth.