I work with a 14 year old boy who is a biter. He bites when startled by loud noises or intrusion into his personal space. But he also holds 'grudges' against children who have screamed days before and may bite them on a day when they are quiet. He often avoids tasks by asking for a break and the last time, I tried using 'first/then' with him but he got too agitated and bit.
When he bites, it is hard. He often breaks skin. He seems very angry when he bites. Social stories do not seem to be helping him.