Surely it would be Dutch. The one reason? I speak it!

Sindarin

I think Spanish is probably the easier choice. Since you are looking into starting so young, I would go with Mandarin Chinese. A lot of school systems still offer French and Spanish, but there isn't that much general use in knowing French, so if your schools don't offer Chinese, your kid can take Spanish in high school, and know Chinese from your efforts.

Immersion is always the best way to learn a language, but that is clearly not an option for you. In general, the older you get, the harder it is to learn a new language. I would think that checking your library for conversational audiotapes would be a good place to start, it will give you some audio to work with, but no text. That said, they are almost all terribly boring, especially for a nine year old, and the best bet for a good incentive for a kid would be a text-heavy import game. I can't think of any Chinese-language games, but I know there was a series of DS games that were advertised as language tools, they looked like interactive versions of conversational audio tapes. Maybe they have a Chinese one?

C#

Pig-Latin

hyway ustmay ouyay skaay

Chinese or Spanish.

Chinese if you want him to be a spy or counter-spy.
Also consider Arabic, though 15-20 years from now the middle east may or may not be the next Soviet Union.

I'd also ask your son if he wants to/what he'd prefer. I'll never forget the day my dad picked me up from a friend's house and asked "trumpet or saxophone?" I answered saxophone, and was promplty given a clarinet. Ah, the power of choice...

ENGLISH, MOTHERF***ER, DO YOU SPEAK IT??

But seriously, I'd have to agree with the Spanish/Arabic/Chinese mentioned earlier. All of them are spoken far more commonly in the world than English is, and all seem to offer many opportunities both presently and in the future. Although as Floris mentioned, Dutch >>> Everything. But that's just because we're so damn cool.

Latin? Why not Sandscrit while you're at it? Chinese or Spanish would come in handy. You can teach him Arabic only if you want him to be pestered by the CIA to "help them out" for the rest of his life.

Spanish. Chinese is probably the sexier choice, but it's tough to learn a language with the promise that he'll someday need it. Plus, I'm not sure learning Spanish in high school will necessarily translate to a useful grasp of the language.

Although Mexican Spanish is the closest to Castilian (Spain) Spanish of all Latin American Countries. I took Spanish in high school but learned mostly through my Dominican friends in my neighborhood. Working in restaurants all the Mexicans thought it was extremely funny to see a Japanese guy talking like a Dominican. Good times were had by all.

Mandarin is a great language to know for business, but something like 2/3 of China speak a dialect of Cantonese as their primary language. I know growing up in New York City it was very rare to hear someone speaking in Mandarin as opposed to Cantonese.

Probably in that order, definitly. Chinese, Spanish, or German. Although having finally travelled to Europe, many people speak English all over the place, except for security guards at Charles DeGaul (sic) Airport. Those snooty f*cks.

I am by no means an expert or even very knowledgeable about the two but from my experience they are very different. Basic grammatical structure is the same but the vocabulary is completely different. Cantonese uses very hard tonal sounds while Mandarin is much softer. All the Chinese friends I had spoke Cantonese and couldn't speak or from what I could gather understand Mandarin very well.

Cantonese uses 7 tones (technically 9 but no one can really distinguish the last 2) and Mandarin uses a subset of 4 of those tones. Cantonese is just more dramatic......

Any language. The idea that this is a language the kid may have to speak later because you live in LA and "need" Spanish, or French because you live in Canada, is not the point. It is about keeping those parts of the brain that deal in language acquisition and retention active.

Think of it like having the kid picking up an instrument. The point is not if he/she will some time be a concert pianist or be playing Flute for the Orchestra, but to keep the mind and body active and honed for those like skills.

A language is about keeping those areas of the brain sharp for retention of new vocabulary. It is about keeping the mind open to learning new languages. The more languages you can speak, the easier it is to learn a new one. In the age of global business, you never know if you will need to speak Mandarin, Japanese, or Hindi for that matter. So being versed in a particular language pales in comparison to being comfortable with language retention and education.

Potty!