Our dog goes through phases where he won't go outside.?

we have a 2 yr. old white german shepard and a 12 yr. old blue healer mix. the shepard goes through phases lasting a week or two where he won't go outside (in our back yard), then he seems to be ok with it for a few days. we can't seem to figure out what triggers this "strike." he will get close to the door and then we have to shove him out or step out ourselves to get him to go, but these things don't always work. he weighs 80 lbs. for some back info, these dogs were left alone a lot for a period of about 6 months or so, before i moved in. the older dog is well behaved and mellow, but the shepard has had some dominance issues. they get along well, occasionally having a small tiff. is this just a matter of taking this dog on some long walks? some part of the dominance thing? any ideas about this?