Need suggestion

I am sending 2.8 months old toddler to preschool from February. He is not sitting in one place and Sometimes he is pushing his friends. I consulted with doctor also.doctor told me to wait for 3 months and make him sit at one place . I am explaining him to every day and trying to sit him everyday . I told preschool teacher that if he push his friends stop him and separate him from others kids . It will take time to improve in his behavior now teacher is complaining he is not sitting and he pushes sometimes. What should i do? Its okay to complaint sometimes but affects my health also. I could not concentrate on my work or his activities