Say OUCH! Make him know it hurts you, make a face, cry whatever you have to.

At 12 months he is old enough to recognise "hurt mummy" or "upset mummy"...say ow, make a face, take him off straight away, be upset with him and explain that biting is NO and will cost him the feed.
And then don't offer it again.

Charlotte only bit me a few times at around 10 months, because she soon got the message that biting mum was not a good idea.
Not sure where this notion of not saying ouch etc comes from, I suspect for younger bubs it can seem a game, but at that age he should recognise unhappy mum and understand that hurting you is not cool.

GL!