Who diagnosed your son? Was id an family Dr, psychiatrist, a psychologist? Please get a second opinion, from a mental health professional if your son was not originally diagnosed by a mental health professional. 5 is very young to be diagnosed with ADHD and medications can be very harmful to children when taken long term.
I don't want to preach, but I see it all too often as a pediatric psychologist (in training). Kids are being over medicated and over diagnosed with illness when they have a behavior problem that can be fixed without meds.
I have seen many a child benefit from therapy and behavior modification with no medication involved. I don't know your entire history, so this is just my opinion in general. Medication should always be a last resort, not a first line treatment.
Good luck, I wish the best for you and you son.