QUOTE(x-music-fairy-x @ Apr 23 2009, 05:24 PM)

I agree with not play the instruments in the same band/group as this would cause confusion never knowing which instrument you are playing (from my experience this is). Also just an opinion of mine but the tenor horn does have a lovely tone!
I agree with this! I turn up to my concert band every week with 5 different instruments never sure which i'll play! Usually I'll sit there with all of them out depending on what each piece calls for

This can be highly annoying but at the same time it keeps things fresh!
I can't be bothered to quote things so I'll just make a general reply to most things that have been said.
I'd tend to agree with your son's teacher, Dora. Your son is only grade 5 so he still was quite a long way to go until he is secure in his technique and his embouchure is 'fully' developed.. Learning another brass instrument would only slow this development and possibly prevent him from being a really good cornet player. Learning two at once would rather be a 'two steps forward, one step back' situation with the risk of damaging the cornet tone on the way. I know quite a few brass teachers who play a whole range of brass instruments from trumpet to tuba to french horn, however, all of these first learned their instrument and became a really good standard on that before venturing to the other brass instruments.
Though I will say that he is lucky if he decides to learn a lower brass instrument as from the people who I know it is evident that it is much easier for a trumpet/cornet player to play on a larger mouthpiece than it is for a trombone/euphonium player to play on a much smaller mouthpiece.
Don't be mistaken, although the trombone and euphonium are 'similar' and can be played with the same mouthpiece (I use the same mouthpiece on both trombone and euphonium) they are both different beasts, and both require a different technique! Obviously one has a slide and the other a valves there are other differences. Tonguing technique for one is different; if you attempt to play legato on the trombone you end up with a glissando. Once again, as Kenm said, the euphonium requires a lot of air and so you have to get used to this also.
If he does decide to learn another brass instrument, then perhaps hold back on the trombone for a little while so he can get used to the euphonium first. Then again, as I type this I'm thinking it'd be best to forget about the euphonium for now and just go straight for the trombone! Although the initial progress will be slow due to the slide I feel that it'd be much more benefitial! Your son's teacher is a trombone specialist so it'd make more sense to choose this as an instrument because later on you'll notice much more of a difference having a specialist teacher than learning the euphonium from someone who doesn't specialise in it. I found this having a french horn specialist teach me trombone - as a beginner it didn't really matter, however, as I progressed it was clear that there were holes in her knowledge of trombone technique and that i'd be much better off with a trombone specialist.
What would the purpose be of your son taking up the trombone/euphonium? If it's to provide more musical oppurtunities then the trombone is the one to choose, as others have said; there isn't much room for the euphonium in an orchestra. Though, as Kenm suggested, looking at another family could be benefitial as it would open up playing oppurtunities in Orchestras, Concert Bands and Jazz bands - dependant on which instrument he chooses of course. It's a shame that he didn't like the violin as there are always playing oppurtinities for this in a lot of amateur orchestras - especially university orchestras where the strings often go unauditioned.
You say that he will be able to play in his various groups on the euphonium straight away, but surely he will already be playing the cornet in these?
Sorry if the post is a bit jumbled - I'm tired