Couple of thoughts.
the stuff in the school restroom could have been because were he wanted to go was occupied and he had no other choice. An 8 year old is pretty low on the totem pole when it comes to using the facilities. I wouldn't worry too much about that. although, I do wonder how people found out and reported it. Obviously someone was in there, which makes me think he was forced into the situation.
Most ADHD kids, I think, know right from wrong. Its just that they don't always think about it due to their instantaneous actions and lack of filters. Since he was at his grandfathers, I wonder if he was off his meds? That would explain a lot - and is why margypops made her good recommendation.
It may be a good idea to take him back to the prescribing Doctor and ask about the dosage and if the meds need changing,in my opinion they are the cause of the behavior..good luck
Strattera is used to treat ADD/ADHD in both adults and children. However, Prozac is used to treat depression as well as to eliminate or reduce the symptoms of obsessive-compulsive disorder (OCD) and bulimia nervosa (an eating disorder). It is not used to treat ADD/ADHD. Something is missing here .....