I am assuming you are working so your child must be in preschool?
I would check with the preschool to make sure this is where you want your son. children at that age generally do have a hard time making the transition, but if the school has good teachers and the classrooms are run properly, I would have an appointment with the teachers to see what may be triggering his behavior. Or it may not be the best school for your child. In that case, find another school. I have a hub about finding the right preschool for your child.
Before considering another child, I would make sure you and your husband are ready to bring another child into your life. Your youngest one will continue to grow and mature, but most of all, are you and your husband ready to parent two young children?