Its hard for me to offer an opinion on if the puppy is traumatized or not without both knowing him, and being able to observe him.
However, I think you are correct to be thinking about these things and trying to adjust for the right amount of "input" for the puppy. Yes, you certainly want to be socializing the puppy. He sounds a wee bit timid. But next time perhaps you can arrange things so he gets to play with one or two puppies, rather than a dozen. Its good that he seemed to gain some comfort level as the session went on. That indicates he was not really over done a lot. Its also important puppies get rest and peace and quiet after such a session, as they will be very tired and recuperation time is important to sort of "absorbing" the positive lesson of the play date.
In so far as grooming, that is a different issue and I hope one of the groomers here will comment on that. Grooming tends to be very traumatic for a dog. Most often what they do in response is to go into a sort of state of "shutdown" in which they may appear calm, but actually they are sort of paralyzed into inaction. An hour and a half is a very long time for a puppy (or dog) to be in a fearful situation. Next time, I might choose not to leave the puppy, but rather find a groomer that will let you stand at the puppy's head and feed him very tasty treats while the groomer does perhaps 30 minutes of something very easy on the pup, such as just brushing. A groomer SHOULD allow you to watch. If you are not allowed to stay there, then I would not leave the puppy. And be ready to call it quits for the session if the puppy shows signs of stress.
I think you and your BF are very sensitive and caring and should trust yourselves in terms of learning to read your puppy and how he is feeling. The fact he seemed to get "too" quiet after all this input may indeed mean he was over-stimulated. Or maybe he was just tired. But from what you describe he is a little timid by nature, so you want to be careful. He needs lots of regular socialization, but in very short, positive, not too overwhelming sessions.
Go over to this thread
and learn about calming signals. I think that will help you to read his stress levels better.
There is also
which is excellent, not too long, and has lots of pics to help you see this in dogs. Some of it is very subtle, so not what we humans normally notice without some training for us!
Hope some of that helps.
One final word... puppies are very resilient, and none of us have done a perfect job raising one. So don't be too hard on yourself. Just keep learning and you'll continue to make adjustments. You are already 100 steps ahead of most people who write in here!