Dear Maria,
Biting among two-year-olds normally does cease when an effective plan of behavior management is implemented. It sounds like you've done some good things, but you (and the daycare setting) might fine tune the process.
At the outset of group activity, it's important to underline that biting is not allowed. At the first sign of aggression, your son should be placed in time out, with a simple statement: "No biting." A timer should be used to track his time out period (several minutes will suffuce). When the bell or buzzer sounds, he should be allowed to resume some activity, but by himself for a short time, again with the instruction: "No biting." Only after a period of succesful quiet play should he be permitted to join more children.
It may be that the daycare setting, with its demands for interaction, is a bit overwhelming for him. It is quite an adjustment from what he's experienced before.
It wouldn't hurt to have him observed by a child mental health professional. Such observation can pinpoint the pattern of behavior and give you some guidance re: the issue of normal-spectrum child behavior problem vs. emotional/psychological problem, and can yield some hopefully useful guidance to daycare staff.