At 14 weeks, puppies can be going through a "fear period" when suddenly things like meeting other dogs can be worrying. The best thing to do would be to book your pup into a puppy socialisation/training class as soon as you possibly can so he can get to meet new dogs in a safe, controlled way. You also should be careful not to reinforce your pup's fear by picking him up or comforting him when he is nervous- this tells him he was right to be frightened. Try to ignore nervous behaviour as much as you can - just jolly your pup along with a cheerful voice, but never reward him for being afraid.
