Dog behaving unusually. Depression or needs vet visit?

My male Cairn Terrier 6 year old has been acting different these past couple of days. He hasn't been coming in the kitchen when I'm eating (I give him little bites of my food) and doesn't want to go play in the backyard. He just lays in his bed in the bedroom. He did go on his walk but wasn't a excited as usual. I'm wondering if he's depressed or if I should take him to the vet.