One thing that is interesting is that when I said "Afuera", at first he hesitated. The reason was because I didn't say "outside". I was under the impression that he did not understand "inside" and "outside", and perhaps he doesn't, but he knew it was one of those two words. Pretty interesting.
Still, he is getting "afuera" pretty quickly. He trots outside like no problem. "Adentro", though, still seems to be confusing him sometimes. He hesitated, and still does once in a while, but only on "adentro".
I am not sure why that is. I have to use a hand signal to gesture him inside when I say the word. Perhaps he can't hear the word too well, or perhaps I'm not saying it consistently (entirely possible, not a native Spanish speaker and just learned the words myself).
One day I will need to practice sending him in and out, and perhaps even use the clicker to mark the correct behavior to help him really understand.
Then after that, it will be time to give a "test" to see where he is in understanding and where more practice is needed.
So far, I think he's learning pretty well overall. Seems like using Spanish is still a good way to expand his "vocabulary", and introduce new concepts or reteach old ones.