Hi Rima,

My son (3) went through a similar stage a few months ago and most moms I know also say that at some time between 2 and 4 their kids got through a spurt of hittng or biting when they're frustrated instead of using their words.  

Even though it's normal, I think you're right to be taking it seriously and thinking about ways to get him through this stage as quickly as possible!  I think the most important thing is that his behavior is dealt with in a really consistent way.  I'd talk to the school about how they're disciplining him when this happens and then (assuming you agree with it!) make sure you're saying the same kinds of things and taking the same actions at home.  With my son the most effective thing was removing him from the situation when he hit (making him leave the park or playdate, taking him out of his class for a few minutes so he missed a fun activity, etc).  At first we'd give him a warning "If there's any more hitting we're going to have to leave".  That helped but didn't completely fix the problem so we adopted a "One strike and you're out" policy.  If he showed any agressive behavior we'd leave right away.  It was hard on all of us but he really worked hard to find other ways to solve problems and now (almost) never resorts to hitting.

Hope this helps and please let me know how it's going!