Keeping Your Dog in the Yard?

So I have this beautiful boy; he's a two year old German Shepherd and the past few months we've been having a lot of trouble keeping him in the yard. He's been fixed for over a year now and we live out in the country, but on a hwy. We own 5 acres of land and the yard is fully fenced except for the entrance. He's been going over to the neighbor's as they own an unfixed female German Shepherd. My question is how do we stop him from going over there? I've been keeping him in his run and letting him out a few hours a day (I have to really watch him), but he still goes over there.

We play with him, and have two other dogs that he can play with yet he keeps going over there. Even if he's fixed will a male dog still wonder away if a female is near by in heat? I feel like an annoying neighbor because my dog's there more then I'd like. I go get him the minute I notice. He's friendly and all up to date with his shots, but I also worry he'll get hit one of these days. I'm running out of ideas and need any kind of advice.

By Jackie S from Northern Alberta