If I was you, I would give the parents a set date to see an improvement and if not, then termination.
Also, if he is doing it at times when you can't give him all your attention, then I would put him somewhere where he can't get to the children. So, either put him in a playpen or strap him in a highchair. I had two biters, and this is what I did at any time I couldn't keep my eyes on them and shadow them. It did curb their biting for a while and if they started bitin again once I stopped containing them, then I did the containment for a couple of weeks until they understood that they don't get to have fun because they bite, etc...

Also, whenever they bit, they got seperated from their friends and had a time out while I paid attention to the child who got bit.
I was also up front with the parents that I did this, and they were fine with it.