It might be that he missed an important developmental period to comprehend language if he was in a shelter.
Yes, this is my feeling as well. Of course I don't know Carbon's background for sure, but from what the shelter told me, he was in two different killing stations before the shelter. He is also clearly younger than the shelter thought (younger than what they put as a guesstimate age on his passport), which means he must have been surrendered to the killing station very young - my guess would be around 10-15 weeks. As far as cages/runs, the shelter and the killing stations are very different, but in terms of human contact, there is very little in either setting. So...makes sense to me that Carbon can't seem to wrap his head around language.
I do find it sad, but more for me than for Carbon, because I'm so chatty with my dogs and my Rottie boys were both very good at language. I was very proud that Duncan knew the names of all his toys and would go find them by name. However when Brogan went deaf as an oldster, he learned everything by hand signal very nicely and I decided to keep on talking to him regardless just because we both liked it.
I'll just keep chattering away with Carbon as well, but it has made me think about training and the changes I need to make in myself to better meet his non-verbal learning style.