I'm 63, and I assure you that toddlers bit when I was little. In fact, I was a biter – only once that I know of. And a younger sister repeatedly bit me and the next oldest, according to my grandmother. As I grew up, a number of neighborhood toddlers developed reputations for biting.
Biting is common – I'd guess that most preschools have to deal with it somewhat regularly. Any child can become a biter, a practice that usually emerges between the ages of 1.5 to 2 or so, and as your school mentioned, it's usually because they don't have words yet to say what they want or tell another child to stop bothering them. They bite out of frustration or sometimes too much excitement, like puppies nipping.
Shadowing known biters is the best accepted professional response, does prevent most biting, and is usually effective within a few weeks. But even with shadowing and teaching the kids to use words instead of biting, bites happen very, very quickly, and not every bite can be stopped in time.
It is distressing to discover your son was bitten, and I can really understand your upset over this. It is equally distressing to learn your child is the biter (my grandson, a generally agreeable and gentle child, had a few weeks at his preschool being "The Biter.") If you did switch daycare facilities, you could again encounter biting toddlers in other places as well.
There is some possibility of infection, but most bites don't break the skin. You can help resolve the swelling and bruising by applying Arnica Montana and/or St. John's Wort (also called Hypericum) oil a few times daily. These herbal oils are available at your local health food store. They work best immediately after the injury, but will help the bruises go away faster even a couple of days later.
Sorry to hear this has happened to your little guy.