It's so sad to read about puppies being fearful - they should be rolling around on the floor having a blast....being a puppy with no care in the world
I agree with everyone - just take it one day at a time and let him come around ...I hope they weren't too abusive
...plus he just endured a plane trip too and imo - that would be a little traumatizing for any dog - especially one who possibly had no human contact
good luck ! I bet with lots of love and patience he comes around and welcome