Hi Gina
6 wks is early, but not THAT early (7 wks is advised). The pup should still have had some time to socialise with other pups in the litter, so I don't think I would be blaming the age you brought him back for that (although, along with other factors, this could have played a part).
I'm also not sure what good it is to find out what the reason is - he's the way that he is, and we have to work with how he is now. It could be nature or nurture, or both, but focus on fixing things, not finding the reasons.
What methods did the trainer use in the class you went to? "Dog whispering" is not really a very well respected method! You need to find a trainer who understands learning theory. My advice would be to find a clicker trainer in your area. Clicker training uses only positive reinforcement (treats and toys) and no aversive methods.
I'm assuming you live in the US? Whereabouts? If you have a look here you will find a list of trainers which use positive reinforcement: www.apdt.com And if you look here, you will find a list of clicker trainers in the US: <a class='url' href='
http://www.clickerteachers.net/'>http://www.clickerteachers.net/</a>
I think the answer for you is to continue to go to classes but to work at a distance away from the other dogs that Oscar feels comfortable at. Don't move any closer to them until he is fully relaxed at that distance, then work slightly closer. It might take many weeks or months, but eventually he should be able to work close to them without freaking out.