Does my dog have separation anxiety?

My 8 month old rat terrier is such a well-mannered pup. I'm living in a house with quite a few people and other dogs and he just loves it. The problem is that when I (specifically me) leave and come back, I'm told that even when Rider (my dog) is laying/sitting/cuddling, etc. with someone - after 2 minutes he starts to whine then runes to the door checking for me. Why is he doing this!? He gets a lot of attention! I'm moving to an apartment soon so I'm concerned that he'll get worse. What can I do to ease his anxiety!? Thanks.