Of all the words I've taught him so far, I don't know why it didn't dawn on me to tell him he did well! I mean, of all the things...something as basic as "good dog" or similar escaped me...D'oh.
So I'm on the way to rectifying such an oversight.
Next Spanish word he's going to be introduced to is "bien" which means "good" in the sense of describing something as good (like good food) as opposed to how something was done (like good fetch).
The question is..which? Well, it doesn't really matter to him (he doesn't know what meanings to words are anyway. I could tell him "grass" for all he cares) but it would help me remember. I think I'll go with "Bien" because I'm describing the thing he just did, though "Bueno" sorta rolls off the tongue. Argh. I might just teach both!
The method, no matter which I pick, will be the same as when I taught him "good boy" or what the click from the clicker means. I will just say the word (say, "bueno") and then give him a treat. That's it. Then when I use it in the "real world", I'll say "bueno" and then give him a treat. I know he has it when I can say "bueno" and he immediately looks for the treat to appear.
It's been a while since I've taught him a new Spanish word, so this will be fun.