child having trouble at school

Hi ladies!
My son is having some trouble at school. The teacher pulled me aside this afternoon and told me that she had to keep my child in at lunch because he did not finish his work (she let him eat his lunch but then he had to go back into the classroom after he finished eating), which i am completely fine with . He is very slow when he is doing any work at school even if it is something fun. He seems to go off into fairy land all the time. I have noticed the same issue at home as well. This has become a big issue at school because now he is beginning to fall behind. I know he is having trouble with math but he is a great reader. I am getting frustrated because i dont know what i can do as a mum to help him focus and keep on task at home and at school. If any mums have had the same situation can you please tell me what you did to help your child.