Yes, I agree with Margypops, this might be triggered by something. Sometimes something happens at school and the kids esp boys don't verbalize it and instead resort to physical means. I've had this issue with my son too. He's 6, but it's like when he's upset, he forgets how to communicate.
Hi nihallia Your son sounds upset by something, has there been any new happenings in his life , new sibling, move of house or school, change in seeing close family, chilred do react from frustration when new things happen in their lives. Have you asked him what he is feeling, why does he hit his friend , is the friend kind to him are you sure it isnt both of them and your son gets the blame , ask who starts the issue .If this has come on suddenly I would definatly say something has triggered it, so check out all avenues. good luck ,let us know how he is doing .