Going to dog sit soon...?

for family... they did a trial run for two days at a dog kennel. and it was not a good experience, the owner was afraid to handle him and he stayed in a cage for 2 days...
he is a large 1 yr old german shepherd..
the dog know me, and i said i would stay with him at their house and dog sit. also take along my almost 4mo old lab female..
they have been introduced. and i will gostay for a few days at their house before they leave just for more family/social time..
but yesterday i noticed his behavior has changed.. when they leave the room he is really stressed until they come back...
he was never like that before...
and before i could give a command and he would listen to me even tho i am not his "pack leader.."
my question, is how will i deal with his new seperation anxiety?? we will be at their house i think will help.. but what else can i do??
Answers:
try getting a tape or cd recording of their owner and play it while you're at their house. Just hearing their voice will probably calm him down. Maybe even playing with him and getting him excited and playing then maybe you could get him to do things like fetch or other games and maybe he wouldn't be so anxious to see them again and will be less stressed.

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As soon as they get out the door and head south, leash him and get out the door headed North. Walk him briskly with NO slack on the leash. Don't take your dog yet.
If he begins to show signs of aggression to the your dog bring them together leashed. I just had to do this with two of mine. One was aggressive. I leashed the aggressive one and called the other over, as he began to display signs or EVEN LOOK AT the other dog I corrected him. No verbal commands, just "AT!" once he calmed down,(after several times), a simple acknowledgment praise was given, "goo'boy" Haven't had to break up a dog fight since.
One thing you MUST understand . . . YOU ARE his pack leader. All humans are above all dogs in the pack. If you aren't . . .TAKE OVER . . . for your and the dog's sake.