Expontential vocabulary expansion mode

It has been a VERY eventful week for Michael.  After getting better (about Thursday, 8/11), Michael has been in exponential vocabulary expansion mode.  It's really an incredible thing, watching him go on a vocabulary renaissance.

He will pretty much repeat whatever we say to him, to the best of his ability.  His current vocabulary:

Mandarin

• He will count to 3 by himself, unprompted, and then look up at the ceiling and think and think.  If I give him a few more numbers, he'll pick back up and finish counting to 10.
• mei-guo2 (America)
• zhong-guo2 (China) He says both "America" and "China" unprompted, and randomly.
• zhi1-dao3 (I know/I understand)

Cantonese:

• yuk6-yuk6 (meat)
• choi3-choi3 (vegetables)
• pa1 (scared)
• mei1 (not done, in response to "are you done/not done taking your bath?")
• yai1-yai1 (misbehaving child).  When he's being lectured for misbehaving, yai-yai is like saying "bad boy".  He'll repeat it.
• saam1 (clothes)

English:

• Mommy
• Daddy
• Car (he LOVES cars)
• Shopping
• Meat
• Up

For some reason, he's most apt to pick up vocabulary in Mandarin.  At first, I thought this to be really odd.

My first language is English, second is Cantonese, and then I speak a smidgeon of Mandarin.

Wife's first is Cantonese, second is Mandarin, and third is English.  (she's fluent in all 3)

Since I really don't speak Mandarin, we communicate in English and Cantonese at home.  However, we both want him to speak Mandarin.  I think he really takes to Mandarin, since that's almost become his "educational language."  Also, since it's nobody's first language in the house, whoever is speaking to him in Mandarin is trying extra-hard to speak slowly and enunciate.

He really hasn't taken to English all that much.  To be honest, since English is a non-tonal language (unlike Mandarin/Cantonese), I think it's less "fun" for him.  He likes the sing-songy ebbs and flows of the tonal Chinese dialects that we're teaching him.  It's really cool, he really seems to "get" the tones and how they confer meaning.

I know he understands a lot of English, because if I give him an instruction "stand up", "turn off light", he'll go and do it.  Well, sometimes.  I know he understands, but as everyone knows, there is a big difference between "understanding Daddy" and "doing what Daddy asked."