It is fear and stress most likely and lack of social skills.
I would give him time but I would limit his access to new people for a while... yes I said it lack of social skills but he is worried all these new people are not going to respect his need and they do not know him.
He may have picked up that one of the new people in his life is not safe.
The more he practices it and you worry and reacted to him going off the more ingrained and the harder to change the behavior it becomes.
Give him time to get adjusted to his home but your worried about a bite then it time to call in hands on help. Someone that can actually see what is going on and see the body language and know if it is fear or true aggression.
If it is fear any thoughts of snapping him out of it by force or using water bottles or cans of pennies or rolling him to show him who is boss will only reinforce that the strange person coming in the door is very worrisome and should be bite as he causes his Mom to do sacry things to him.
Suggest you call one of the following and see it they work aggression or have a local recommend for fear and aggression
Washington
Diane Garrod
Canine Transformations Learning Center (CTLC)
Langley, Washington just 35 minutes north of Seattle
Email:
garrod@whidbey.com
September Morn
Dogs Love School
Shelton, WA
Grisha Stewart, M.A., CPDT
Ahimsa Dog Training
Seattle, WA
JL