Hi, thanks for all your messages.
We are back, and he did it!! (And I had a lovely coffee and a shop 
)
We had booked seats by the aisle and in the dress circle (incidently friend who booked mentioned why we needed aisle seats and we got a 'carer goes free' ticket, not done that before as feel a bit shy about it, but they offered!). This avoided the audience participation too.
Never been to that theatre before but it was fab - big space at the back and the lady there said yes of course DS could come and stand or hide there if need be. She also said there were spare seats futher back for any children who found it too noisy.
DS came out buzzing and had really enjoyed it (he is 9). DS used his ear plugs but not all the time. At one point he had to go out because he was scared, but there was a monitor in the bar so he could see that bit of the show from there.
So proud, because he actually went back in after that bit. It was well worth it, despite the fact that DS had been so anxious beforehand he really kicked off and kicked and hit me and DS1 badly before we left home. My arm hurt all afternoon (need to try and look out that thread with restraint tips).
For anyone thinking of going, DH said the following: it is not full of bangs but there is a loudish noise at the beginning (more a crack than a bang, someone is asked up to press the Start button). At the end of the first half there is a loud bang, but there is lots of warning leading up to it (DS was out of there!). Half way through the second half a hydrogen balloon pops, this is more a flare than a bang though. The rest of the show is fun, silly science stuff, and quite a few TV reports from various parts of the Science Museum.
Hope that helps anyone else thinking of going.