D.K.
I had to laugh when you said you tried talking to him. He will retain very little other then it is not okay. I watch little boys so I know from experience. When he is older you can explain you understand his frustration and validate it however give very clear rules of what is appropriate as far as expressing it.
I don't agree he saw someone else do it necessarily and I don't agree sign language is going to help him at daycare if the other kids don't know what he is doing. Kids lash out when upset at this age, either with hands, fits, biting, legs or whatever as they don't have the words to use when they are upset.
I would have consequences for him at home as well inform daycare to have them too. Remove him from the situation and put him in time out alone. He will get it eventually.
Boys lean more to being agressive and physical when upset or frustrated, teaching self control takes patience and time but to be honest he does not have what it takes to "talk to him" about it. I can talk to my almost five year old son and at some point they just look at you like they are so over what you are saying. hee hee, we call it selective hearing. I have him repeat back to me what I said too so I know he heard me. The best way now is to give very simple and clear rules/consequences. We do not bite! We be nice to our friends. If you hurt your friend you will be in time out!
Set him off by himself for three minutes...then when you get him from time out remind him what he did and it is not okay, then thank him for doing his time out. It doesn't matter if he is fighting back, the teacher should be aware of when it is happening and try to intervene before it esculates.
Make sure you are consistent with the daycare too. If you both have the same consequence he will get it faster!!!
Biting can leave serious marks and some parents could raise a stink about their children coming home with bite marks not to mention if it continues they can remove your son from the daycare. I witnessed this first hand when I worked in a daycare center.