Awww, poor Jack! Can you keep him on a leash for some supervised time with them? He may be afraid of them because they are so much littler than him, but getting him used to them now would be a good thing, and let him know he is still first dog in your heart! If you can wear the puppies out a bit, then have them interact with him when they are not quite so hyper, it might help him be less nervous around them.

Most of all, time will be the answer, but you can help him be less afraid now, too!