My 2 year old son started taking Singulair chewable tablets last ...

My 2 year old son started taking Singulair chewable tablets last year. I noticed that his behavior changed every day after giving him the medicine. He would become easily excited, run around the house, and act like he didn't have common sense. When I talked to his doctor about this, she thought it might be from the aspartame in the pills. Since he started using the packet powder without aspartame, he has been fine. The medicine really works for treating his asthma. He is now 3. I am concerned about the recent findings. He is too young to tell me how he feels. Has anyone had an experience with aspartame in relation to Singulair?