What orchestral instrument to learn?

No idea if he is musical, but if he liked it it would be nice to have started on an orchestral instrument. - re-reading the OP again. I wonder if the best start is to find out if he IS musical before deciding on an exotic instrument :).
Personally I think piano is the best one to start with.

Agree with Mistlethrush's wise words - v sound. Another thing you might want to factor in is cost. Something like a bassoon or a double bass will set you back ££££££££££££s, oboes don't come cheap, but flutes/clarinets are more affordable and so are starter viledins etc. [Is it a coincidence that rare instruments are also expensive?] You can hire to start with in case he gives up. but eventually he will need his own instrument. I hope that woudln't stop him fulfilling his dream, but you can at least be prepared. (New car, new bassoon? Guess which won?)

Also I would second all above who've said that orchestras always need violins and cellos (and violas of course - yay). In general choosing something just because it's 'rare' is not in my opinion a good thing but I've said that before and in the end folks make their own choice. (Is euphonium rare? Kids at DC's school tended to start on that and switch to horn, trombone etc. as they grew, but maybe that's a local phenomenon. A bit like the glut of bassoonists round here, but I digress...)

If you play the celeste you'll spend most of your time being a Sugar Plum Fairy at Christmas time...

cor anglais - you're actually an oboe player who has managed to scrape enough money together to get another instrument that you'll play rarely.
alto flute - see above
contrabassoon - see above, with knobs on, and need a forklift (not really, but still....)
harp - a good one to fit in the car - no, you need to fit the car to the harp actually, or buy a van. And then you normally need help from someone else to get it in. Then you spend 30 mins before the concert tuning up. Then you spend most of the interval (when the rest of the orchestra's having a drink and a wee) tuning up again. And you're only needed for one concert in every 20.
organ - great if you want to play in church, but you need to get very good at that before you'll be asked to play with an orchestra as organ and orchestra is used even less than harp and orchestra.

euphonium - good for a brass band, certainly.

celeste - percussionist - or someone else that plays the piano well and is already playing a different instrument in the orchestra. Very limited repertoire, even if it is very obvious when its playing

The trouble with hardboiled's list is they are not needed orchestrally very often, so most schools/youth orchestras don't need them for their repertoire, or rarely, so can do without very well.

When I was in orchestras on the occasions we needed a harp or celeste or whatever, we'd get a local freelancer in if there was no one in the orchestra who could do it.

Hi tricot...
I meant things like...
cor anglais
contrabassoon
harp - a good one to fit in the car
organ
euphonium
alto flute
celeste

but of course they do need strings, LOTS.

FruitOwl - certainly sounds like that !

Oboe? It's like trying to blow into a weasel, apparently
I used to play timpani in a youth orchestra and that was great fun. The music is easy to read too.

There is a book called "The right instrument for your child" if you want to obsess read up about it like I did.

In the end we went with clarinet because DD was keen and that counts for a lot. My subtle brainwashing is obviously working though because she's started saying "when I get to secondary I'm going to swap to bassoon"!

but whatever he learns, make sure you put piano in there - it's a lot easier to go from a treble clef instrument to a large, bass clef instrument in the same family if you can read both clefs equally well.

it's harder to find people who play the big instruments - like the bassoons and the double bass, and the french horn etc.

Voice and instrument connection - rather a red herring in my opinion. I played the violin - I am now a viola player, but I am a high soprano, so my instrument plays a lot lower than I can sing. My husband is a high tenor and he's mainly moved from cello to Double Bass (I don't recommend a bass as you will have to chose your cars around it for the rest of your life).

<a class="break-all" href="http://www.google.co.uk/imgres?hl=en&client=firefox-a&hs=NL0&sa=X&tbo=d&rls=org.mozilla:en-US:official&biw=1280&bih=870&tbm=isch&tbnid=XTxn5czaIfDW8M:&imgrefurl=thingstoknowif.blogspot.com/2012/08/you-want-to-begin-playing-instrument.html&docid=3P0hG8AD12zriM&imgurl=3.bp.blogspot.com/-V8EqkNT5zb8/UDpO0KqoZlI/AAAAAAAAABU/U9DMAh6JtJc/s1600/orchestra%252Bchart.jpeg&w=530&h=422&ei=JNjuUPTlHYGg0QWw1oH4Cg&zoom=1&iact=hc&vpx=607&vpy=162&dur=4&hovh=200&hovw=252&tx=119&ty=114&sig=102457783427828564122&page=1&tbnh=144&tbnw=181&start=0&ndsp=29&ved=1t:429,r:3,s:0,i:118" rel="nofollow" target="_blank">a diagram of an orchestra

larger nostrum end?
Spell check gorn mad!
Instrument!

How does he sing?
Has he a higher pitch voice, or a lower one.
Also can he hold a tune, Or is he more rhythmic?

If he has a low voice, then a larger nostrum end will be better for him, if he has more of a soprano / tenor voice hell enjoy the lighter instruments - flute violin etc.
If he's not melodic at all, but has good rhythm, he may do well in percussion.

Good luck!

I second flowery with the viola recommendation, (if he can get his head around the alto clef) and definitely a string instrument. Orchestras need lots of string players and only ever need two of each wind instrument and two to three brass, so despite the initial difficulty there is much more chance of getting into things as string player.

Violins are pretty essential in an orchestra - and cellos come a close second. If you learn the violin you can transfer to viola at a later stage (and tbh, small violins are bad enough but small violas are even worse to try to get a decent sound out of). Wind instruments - well you only need 2 whilst you can have 20 violins or so and 10 or more violas if you can get hold of them....

Rarer ones tend to be the bigger ones so think cello/doube bass, bassoon, trombone, tuba etc but probably not ideal for primary school children
Woodwind instruments definitely give a more instant result (and are less painful to the ear!)

My mum advised me to go for viola rather than violin to make sure I got into all the good youth orchestras.

It worked, I got I sooner than I otherwise would, and go front desk/lead which with the standard locally there's no way I would have playing violin.

Having said that you don't get much in the way of decent tunes playing the viola.

I agree with him deciding though. I tried the oboe first and it wasn't for me at all.

How about a "conventional" violin or cello? Seems like a lot of people go for "rare" instruments for the reasons already mentioned and many orchestras are suffering now with the luck of string players. In DD's school orchestra there are 6 wind players and 2 violins .And a teacher has to play cello as there are no cellists.