K.L.
This is a very natural reaction to feelings of frustration. Your son is just dealing with other children at daycare and realizing that he is actually not the center of the universe and that occasionally, that he will have issues with others doing things that he doesn't like or want. My daughter, 2, has been a hitter, on occasion and has also recently, starting biting now and then... since she has been bitten at daycare by other children a half a dozen times or more. On the one hand, I truly believe in them learning how to fight their own battles, and then, of course, we need to teach them what is appropriate and not and when.
So, my advice... find the book "The Way I Feel" and discuss your son's emotions with him... so that he has a name to call those things inside himself. This will help a lot. Also, give him constructive ways to deal with his frustration and anger. My daughter knows that she has a soft, frog chair in her room that it's "okay" to hit when she feels those feelings... but ONLY that... not me, not her friends, not throwing, hitting, kicking, etc. Just hitting froggy chair. Also, suggest some other acceptable behavior when angry or frustrated, like "stomping feet or roaring like a dinosaur"... these things will help him appropriately deal with his feelings without acting out against others. Also, discipline him when he makes mistakes about what he does. Time outs when hitting happens, talking to him about what has gone on during the day at school will help and just acknowledging that it's hard for him to find words (but helping him do so) to express himself.
Good luck!
K.