I would say neuter him also. Adding another Sheltie will either keep him occupied so he won't want to bark, or you'll end up with two barkers. If you do choose to get another one, everyone needs to be spayed/neutered or you'll end up with fighting instead of barking. You can try the squirt bottle, but some dogs like that. My grandmother had a sheltie and when she would water the yard she would give him a squirt with the hose for fun. He would bark while getting wet, then run around the yard and pull up chunks of grass. Being a kid at the time that was the most hilarious thing to watch, but the yard suffered and the water made him bark so I guess it depends on the dog if the water is a barking deterant.