8 year old ASD boy biting

Since we returned from winter break, my son has been grumpy and despondent and regressed excessively. We started ABA right before then and have noticed a change since starting. Most of his issues are happening at school. The teacher is amazing and even she is confused. He’s yelling, crying (he is not a crier), doesn’t want to engage with anyone and plays alone now (he used to want to play with other kids), and today was biting.

PLEASE HELP. I’m at my wits end. I have a conference scheduled with the teacher next week. Any advice I’d appreciated.