Hello, I'm delurking after forever to respond to this, so I hope I do it right!
I am a preschool teacher and also worked in a childcare setting and I think honestly you need to ask THEM what THEY are doing about it. I know it's frustrating for teachers and other parents to have a child who bites, especially beyond the age of 2. But since he is a bit older and it's not just about exploring the world orally like say, a toddler would, there is something else going on. You are not there to see it, but they are. It makes me sad when kids get dismissed for this kind of thing especially if nothing preventative is being done about it. As his teachers, they should definitely have an idea as to what is provoking it. Ask them to log each and every time it happens: what the situation was, how many kids were around, what time of day, if it was a transition period, his mood before and after, etc. All of those things can then show them what times of the day/activities/situations that are stressing him out and if he is overwhelmed. This makes it easier for them to give him more attention during those times. Is it because too many kids are around? Then they need to pull him from the group when he is overwhelmed and have him do a quieter activity. Is it a transition period? Then hold his hand and take him to the next area. Things like that will GREATLY reduce the numbers of time he bites. We have a no dismissal policy for biting at our school but we are SO attentive to the kids who bite that it truly becomes far less of an issue.
What you said is true, you're not there for the immediate action, so honestly other than talking to him about feelings/what to do when upset, etc, there's not much else in your power that will change it. If they are unwilling to keep a log, I'd honestly find another school who would be better willing to work with him. Sorry, I know that's not ideal. I just don't think it's fair to kids who do age appropriate things to not work with them first before just letting them go!
It sounds like they just don't want to deal with it so maybe another school would be a better fit for him.