Hi Anita
Try and distract your pup with a treat or a toy or talking in a silly voice from another dog lunging on the lead towards him snapping, so he is concentrating on you and is looking at you.
If he has been frightened by the experience then act like nothing out of the ordinary has happened. Don't cuddle him and make a fuss. This can be a big mistake as it will encourage him to be nervous. His confidence can grow by taking his cue from you - and if Mum is acting cool then perhaps there's nothing to be afraid of after all!
Your reactions are important and if you scream and shout and freak out then you may be training him that it's ok to do the same. It doesn't matter what other owners do it's what you do that's important. Try and think first. If it's at a puppy class you can go and pop him in the car and then come back in and sort them out!!!
If he spots something he thinks is unusual on a walk or anywhere else you may see him look up at you to see what reaction you are giving. Don't react!
If he balks at something then encourage him gently but firmly to continue. Try not to pick him up. He has to do it himself one day!
I've said 'may' and 'can' a lot because his own genetics are a contributor to his temperament but the above will help with a lot of puppies upbringing.
Good Luck with yours