I have a boy, same age, who was starting to hit and throw toys a lot. He was getting a lot of time outs and it wasn't working.
I reflected on it a lot and decided to be extra affectionate and 'sweet' to him and it has been working really well. I think he was sensing my stress over him and that I did not 'like' him as much as my own kids and the other dcks. Perhaps he was jealous of the affection I give my own kids. I always give hugs and cuddles to the others too but it's not the same. So perhaps this boy is looking for attention.
I also began giving opportunities for this boy to help me with new things so he can feel some pride in what he is doing. He has very few words so does not communicate well but I can see in his eyes that he is proud of his helping.
He has actually turned into quite a little gem as he now watches over the other boys, who are younger, and gives me a shout when someone is getting into something or climbing where they aren't supposed to. So my eyes are now everywhere :-)
Good luck to you!