Boston Terrier Puppy growls and snaps at people

I have an 18 week old Boston puppy that has been fearful of strangers since he was 8 weeks old. We took him out to socialize him all the time with strangers and other dogs, and took puppy classes. at first he would hide and not not take treats from strangers. Now he will take treats but growl after and if they attempted to touch him he would snap at them. We have taken him to doggy daycare for short periods at a time and he will just hide in the corner or do an occasional sniffing the room. I talked to a trainer today and she said his issue needs to be dealt with by a animal behaviorist and gave me a refferel. I contacted the behaviorist and the initial fee is $225.00 just for the consultation, so I thought I would reach out to expierienced Boston owners first. Thanks in advance, Rebecca