It's hard to tell from the brief description what's going on, but I would also recommend talking to the pediatrician and asking about physiological and sleep evaluation. From what I've read 3.5 is too young to be diagnosed ADD. Many children are very active at this age and then slow down and learn to concentrate after 5 or 6. Sleep can be a real issue. Some children experience sleep apnea that causes behavior issues. Snoring, nightmares or waking often can be a symptom of this. If your grandson is in preschool, have they experienced problems? If the child has developmental delays also, he may be able to get testing and help from the school district preschool special education program.
My daughter was very active, moody and aggressive at that age. I did notice that some preservatives like sodium nitrate (meats), BHT (meats and fried fast food) and calcium propionate (bread) seem to make things worse. I'm not really sure about the connection, but it may be helpful to try eliminating some preservative and colors one by one to see if you notice anything.
One book helped me with my daughter. "What your explosive child is trying to tell you" by Douglas A. Riley. It is a bit for older kids but it might be helpful. Some folks swear by 1,2,3 magic or Love and Logic for parenting techniques.