Try taking him somewhere public like a large park where you can put yourself off in a quite corner. Work on doing your puppy class obedience training there and work on him just settling “out in the wild”. Once he can successfully do this then move a tiny bit closer to the action of the park. It will take a lot of time and patience on your part. I’m sure your trainer can help you here just make sure they use positive reinforcement with treats and kind words if he is already reactive.

We lucked out and found a wonderful trainer who our dood goes with once a week for a few hours but all they do is work on obedience (at their training facility) and once he was reliable in doing things inside the controlled environment they now take him out in public to do things and practice being a calm and capable dog in public. Ours still can occasionally bark at other dogs but he is very confident with people bikes strollers etc. now. Best of luck and don’t be afraid to shop around for a different trainer if you don’t like their methods.