Foster Care Support Group
Foster care is a system by which a certified, stand-in parent(s) cares for minor children or young peoples who have been removed from their birth parents or other custodial adults by state authority. Responsibility for the young person is assumed by the relevant governmental authority and a placement with another family found.

I would ask each of them, independently "what happened?" Do the answers jive? or are they completely different? i would ask for a 3 day in house hospital evaluation on your FS's mental health. I would get your son therapy so he can work out anything which is bothering him regarding the attack. Don't leave it unaddressed. Good luck.