I am adopted and so I know about that and I have a 10 year old son so I think I can offer an opinion that has some backing to it.

Some kids really latch on to their stuffies and that is ok. My son has a stuffed dog that he has had for a few years, he sleeps with him every night. Rover is my son's sibling, best friend, confidant and is always there for him. He knows that Rover is not real but does give him security. Your son has gone through a tramatic event, adoption is not easy for anyone, and he has found his voice using his stuffies. He will outgrow the need to use the stuffies as he grows more and more comfortable with you.

Follow your mommy instincts, you know your son the best. Personally if a therapist told me to go against my instincts I would be finding a new therapist.