He is just doing it top make noise and get a reaction out of I do not think it is from not being walk enough. I think you spend a fair amount of time with your dogs plus he has a playmate. My dogs get barky from time to time and the best thing is a spray bottle. Get a big spray bottle with a good 6-8 foot reach on a direct stream. I got my bottles from the dollar store in the garden section. use 1/3 white vinegar and 2/3 water and every time he barks say "quiet" or some other word to mean shut up. Later on you can just say quiet and he should stop but for now spray him and aim for his mouth. It will get in his eye, nose and mouth but it is safe and most dogs only have to get it a few times before they get the idea. it works wonders on most dogs and if you have to you can increase the amount of vinegar till you find the right mix.