Medication

I have a son 6 years old. He is on METHYLIN ORAL SOLUTION 5MG/5ML.  He is doing very well in school but he is very small and thin.  He is going to be 7 years in September.  He only weight 43 lb. and I am worry about that side effect. I took him to another doc and he wants to put him on Addreall when he was 5 years old. His pediatrician said no.. he is just a kid. Let him be.  At 6 years old we put him on Methlyin because of the school. He tend to day dream in class and not be part of the class. He did wonderfull in school. How do we weight the side effect of growth and his behavior. We need to give it to him so he can focus but I am concern about his growth.  What should I do?