My first recommendation is to trust your gut. Your gut will give you a sense of whether it is safe to proceed or not.
If your child is growling, it is a signal of distress. He is not feeling safe. His sympathetic nervous system has been activated. That means he is in a state of fight or flight. Anybody in a state of fight or flight does not have access to his/her higher brain centers which start coming on line right around the age of two. In this particular state your son’s capacity to benefit from a preschool experience may be limited at this point and time. Brains learn best when there is a felt sense of safety, predictability and a sense of “I am welcome here.” If your son has a degree of trauma in his past, his stress response system may be hyperactivated, meaning his brain may read signals of danger where there are none. Being separated from his primary caregiver maybe enough to put him over the edge. As you practice Hand in Hand listening tools his stress response system will become evermore resilient and ready to take on the developmental tasks of being three. He is very lucky to have you.
All my best,