P.M.
My grandson bit another boy maybe three times over a couple of weeks shortly before his second birthday. His parents AND the caregivers took this very seriously.
He was 'shadowed' at daycare so his bites could be intercepted before they happened (this accepted approach works and changes the behavior, but requires a very attentive caregiver who is more or less dedicated to watching one child during his/her most susceptible times).
At home he'd get appreciation for days he didn't bite, and the repeated message that biting is wrong and hurts people. (He didn't get punishment so long after the event, as this wouldn't have made much sense to him.)
In both locations, he was encouraged to use words and tell the teacher when he became frustrated or anxious, which seemed to be his triggering emotions. Some children also bite out of sheer excitement, like puppies. Nevertheless, it hurts, it leaves marks, and it will often get the bitten child to experiment with biting.
I'd press the issue with the daycare folks. Yes, lots of kids bite at some point between their first and third year. Yes, kids get bumps, bruises and scratches while playing at daycare. But deliberate biting can be stopped, and should be treated with real concern.
Here's one of several good discussions of the whys of biting and how to deal with it in daycare settings. http://www.kidsgrowth.com/resources/articledetail.cfm?id=851 Perhaps giving this to the director to read would be a good starting point for a discussion. As a parent of a biter or a bitten child, I would NOT let this slide.
Good luck. I'm so sorry to hear this is happening to your sweetie.