I went through this with ds1, at about the same age, ds2 but it was more scratching/pulling hair and dd2, who is now 2 and 8 months. Dd 2 no longer bites other children but she does bite her siblings occasionally.
I felt terrible with ds1 as I really felt it was something I'd done and he would grow up a psychopath - he is now 12 and a lovely gentle person. I was in tears about it so many times though, and nothing I did seemed to help.
I would agree with Dr Green that it's something they can't really help - I think a red mist descends on a certain type of child when they are frustrated or feel threatened - by someone taking a toy away, or being made to stop doing something they want to do. I really don't agree with screaming at a child deliberately, or making a child scared of you, or biting back as I don't think this works with early biters, I don't think they will remember. You have to tell them it's naughty and put them on the step, or wherever, then make them apologise. Even if you do that consistently you may have to wait for them to grow out of it, but you will know you are doing all you can do. All of mine had stopped biting by 2.5. With my dd2, I stopped going to playgroups completely for 6 months because I had to shadow her so closely and still didn't manage to prevent other children being hurt. I couldn't face any more humiliation, having been through it twice already, but when I went back about 8 weeks ago, she was fine. I still watch her, though! So perhaps you could look at situations that cause him to bite and see if they can be avoided - perhaps he isn't emotionally mature enough to cope with them. I know it's hard if you work and he has to go to nursery, as my ds1 and 2 did, but if you have a choice, perhaps it's something you could think about.
I also looked at what was happening in the family - because there's quite a big age gap the eldest two children love her dearly and will give her anything they happen to have that she wants - my ds2, who's 6, wouldn't so he was the one who got bitten at home. I also think perhaps dh and I were spoiling her in a similar way as she is our baby, so we all tried to stop doing that.
Sorry this is so long but I really do sympathise and I know how terrible and depressed it makes you feel. It isn't your fault and if people are lucky enough to have children who don't bite that isn't necessarily down to their superior parenting skills either. My dd1 has never hurt anyone and was a quiet, angelic toddler. I'm glad I had her or I would think it was me!