Hi Sweep's mummy,
Tilly had a similar experience when she was a pup. She was very intimidated by the big dogs at puppy classes at first, but our trainer told us that in her experience most spaniels were quiet at first and then rapidly came out of their shell. Tilly spent the first two lessons huddled under our chair, but each lesson she got braver and braver, until now she LOVES classes, it's her favourite thing ever and she LOVES dogs of all sizes. She has had some classes where it's just been her and some Great Danes and she has loved it :roll:
!! If your trainer is good s/he will control the times the dogs are off lead and going crazy. Ours was excellent and made sure at first that small dogs were off-lead together whilst big ones were on-lead. So, just be relaxed yourself and try not to fuss him if he's nervous. If you act cool, he will pick up that there's nothing to worry about.
I would strongly agree with PennyB, finding some well-natured dogs at the park is also really helpful. We found a small group who meet at a regular time and although at first Tilly sat at a distance and observed, I let her come to them in her own time and now she runs the length of the field at top speed to get to them to say hello! Most of these dogs are elderly, Tilly is by far the smallest (and cheekiest) and they tolerate her very nicely! They have been great for teaching Tilly some "doggy-manners".
This is a long winded way of saying I would find situations where he can meet friendly dogs and let him come to them in his own time. I thought Tilly was going to be a complete wuss, but now she is almost too friendly to other dogs (has met a few snappy ones, but always gives new dogs the benefit of the doubt!) I know it's hard, but try to act unconcerned as it's amazing how they pick up vibes and if you're nervous it can make them think there' s something to be worried about. Obviously, I would try to check out that the dogs are friendly first!
Good luck, I do hope you enjoy your puppy classes, they are usually great fun!
Sue and Tilly