Of course that he realize that! Babies seek the most practical way to communicate obviating all the rest. So If your grandson knows that your mother tongue is spanish, be sure that he will want to speak with you in spanish instead of any other language.
In my case what I thought is involving him first in other languages more like "a game". So some words and sentences we'll use in particular contexts, so he realize that the only way to play those games are speaking in english or in chinese (for instance). For that purpose you can use some toys (dolls, puppets, teddy bears) and can
convince him that each of them only understand a foreign language 
Apart of playing
the other thing all babies LOVE to do is something FORBIDDEN. So you can also use some kind of reverse psychology telling that you are studying english or chinese but HE is very little to do that. I am pretty sure that at least 90% of cases, kids will want to participate in something they see only adults do (even learning language). Remember that toddlers always want to imitate the same things that make their parents or grandparents
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I also preferred old "Recent Threads" version
). Besides all my books are with those kind of characters.
