tantrums of 3 year old child.....

Dear docs ,

My 3 year old child is behaving in strange way since two weeks and I am worried. He use to be very happy go lucky before , fond of cars etc , sharp brain , use to name all birds animals , cars by their logos etc. However , these days he doesnt take much interest and gets very hyper if his wants are not fulfilled. He grinds his teeth with force and shouts . Also he wants to hit the person sometimes. After some time he cries and says sorry and is back to normal. He has been having regular cold and cough. He has been eating lot of choclates and toffees and less of food and fruits. Earlier he use to be fond of eating everything. My wife keeps quite busy with job and tries to devote maximum time. He is also less playful with his friends. I came from his school and the teachers also told me the same thing.

Please can anyone help.