Or a different cello teacher with experience of children like your son? Just keep him playing something, or singing? v.v. important. How old is he? My DS plays the cello and has terrible posture/bow hold but still enjoys it.
the fact that the teacher says it may not be the best instrument for him (which I think they very rarely do say) would incline me to look at something different. Can the teacher recommend a different instrument for him?
I think I'd stick with the cello since he is enjoying it. It's going to be a bit of a drop back to start another instrument now. I also think that long term, as he improves, it might well help him relax when he is feeling tense. If you really think he needs to go down a different route, I'd say wind, and something like the clarinet or oboe rather than trombone.
I think though with asd you may still always have some issues with expression though if he is tense. How old is he? My ds1 has asd traits (we never went for formal dx of asd in the end, but he has a huge list of diagnosed issues) and is learning all the time as he gets older, and can actually be surprisingly empathetic at times now.
I think there is a bit too much pressure on kids who have music lessons that they have to be good at it. DS2 (nt) isn't great at the violin, but he loves it. FWIW, he has been learning about 18 months, and now holds the bow so naturally he tried to do an impression of his sister scraping it along the strings, and he couldn't do it! I think it is something that does take a long time to master, strings are fiendishly difficult for the first year, but it will come.
Dh teaches trumpet and has taught a couple of children with aspergers. One of them passed his grade 5. My dh got on well with these boys and loved hearing the detailed bus journey through London that one of them made each week! It's interesting that the teacher has mentioned the instrument not suiting your ds. I may be reading too much in to it, but does the teacher get on well with your ds? Could it be that the teacher is worried that the rate of progress might be slow and that they'll be blamed for it?! I know from dh that some parents have a habit of blaming the teacher - not suggesting you are that parent, but teacher might not know that. Not sure how well you know the teacher! When he was teaching the aspergers children, he said there were things you had to just let go...like posture for example. He could see the pleasure they got out of playing even if there were technical/posture problems that he would normally straighten out. If your ds really loves the cello then I'd keep going with it. Is he interested in any other instruments?
We have thought ds has aspergers for a while. School can cope with him at the moment and (hooray) he is currently coping with school, so we are taking their advice to wait a year or so to see how things develop before seeking a diagnosis.
Ds has had cello lessons for a year. He's not going to wow the world with his cello playing at any point in the near future, but he likes it, and I think it's helped his social skills. Even though he's not very good at it (at all), I think it has helped his confidence that he can play stuff.
One big problem is that his posture is very dependent on his mood and he can be quite defensive and scrunched up sometimes. And if he's in this sort of mood he'll want to grip whatever he's holding pretty hard. His cello teacher has suggested that cello may not be the best instrument for him. I can see his point - something which requires fluency of movement, and which is so fine at expressing the emotions of the player perhaps isn't suited to ds.
Are there any instruments where it doesn't matter hugely if you're having a scrunched-up day, and where it doesn't matter if you want to hold on very tight? Any experience out there from music teachers of teaching kids with ASD? I'd really like ds to play an orchestral instrument as playing in a group helps on the social side, but also I want him to play as I can't imagine a childhood without music. Having said that, I suppose music is just one other expectation that may need to be changed to adapt to who ds is and what ds can do and enjoy.