Aggressive child

I have a four and 8 mnths old boy who is turning aggressive day after day. I don't know from where to start but he finds it funny to push us or his friends and hits while laughing *not evily* but like he thinks it's like a game. I tried to explain over and over to him that this is not right blablabla but nothing seems to reach it. He also had some speech problems and he goes to therapy. At first I thought it was a problem of communication but he REALLy understands what we say and doctors said that he doesn't have a mental problem, it's just a speech problem.
Last week when we were at the park, a 7 years old was playing with him and then he stepped on him so my son's reaction was biting him in his ass and the poor boy was wounded (a small one but still there was blood) and today at school he pushed his friend from the slide and poor little boy was hurt too.... I called a local homeopath after the biting accident who advised me to give him 3 drops of Belladonna for three days and we will see in a week... What do you think?