hello doctor

i am seriously worried about my nephew's behavior. He is 5 years, agressively hyperactive, stubborn, started to answer back, and uncontrollable at times. I have a 2-1/2 year old son who sometimes shows the same signs. when both these siblings play with each other, there is lot of aggressiveness seen in the elder one, whereby he unknowingly hits hard and I feel enjoys the act. I have been always thinking of seeing counseling, but was always wondering whether the age is appropriate to start some kind of a therapy. Educational wise, he is bright for his age, but the teachers also have the same complaint; excessive aggressiveness, doesnt listen when he is at that peak, disturbs the class. His positives in the class are neat handwriting and does his work perfectly, faster than others. He is shy in front of new people, yet at home he really takes the nerves out of us. Can you please guide me.