Okay, it's time to start training him. He is currently peeing on your bed probably to say "mine!" but needs to learn HE is not in charge. You and your husband are. The yelling and spanking are actually reinforcing the behavior, not teaching him it is bad. Especially to a terrier, attention is attention - negative or positive. Does he get an after-dinner walk to tire him out (and for him to drain the tank while out)? That might be a good first step. It will reinforce/teach him to walk well on a leash, and learn leash manners, and it will strengthen the bond between you, and it will help reinforce that you and the boss. And it will help tire him out - which with a cairn is always a good thing. He's almost a year old, so he's trying to figure out his place in the world. If you can teach him that YOU are the top of the totem poll, now is the time to do it!

What ages, etc. are the other dogs, and how long have they been around? I notice you said your husband can "let the dogs out" ...