PS If he isnt involved in any yet, get him playing sports and outside activities you are correct he is possibly bored, if there is a Dad get him involved playing ball games and soccer with your Son, get rid of the surplus energy.
Certainly a visit to your Doctor is good if you are concerned about his health. Perhaps you could check why he is beahaving like this,how old is his sister if she is older what is the relationship like or does any teasing go on,flooding a toilet ,was it done on purpose or an accident, The Teacher needs to take control at school and dish out the consequent punishment if he is hitting there, Ask him if he is unhappy about anything at home and at school., it could be he is upset and acting out as children do, therefore before you think he has a problem like ADHD check out other avenues. Put him on a Multiple Vitamin with antioxidants, Vit B Complex and Vit C also Parents say Fish oil helps.