Is he outside alone most of the day? If that is the case, getting another dog is a great idea. Dogs are social animals and need to be with another animal to be mentally healthy.
It would be even better if you were able to spend more time with him.
Would he be happier as an inside dog? Not really, but if he is isolated in the back yard then the answer is yes, he would feel better inside since he would be able to be around his family.
My dogs can come in and out as they please, but as they have a lot of other animals to socialize with they spend most of the day outside. This has nothing to do with weather: this is their choice.