fighting jack

We have a 6 yr old male jack, he is a great dog and minds well other than we can not let him out by himself, we have to put him either in his kennel or on a line leash outside, he will hear another dog and run away even if we are out with him sometimes he listen and comes back but mostly he puts his head down and go after what he hears. We do not have other dogs but we have in-laws that live up the road who have had many dogs we always try get them together to socialized which he is great with playing and having fun, but as soon as their puppy gets out of puppy stage he will not let them near him without attacking them. is there any way to stop this behavior