Orthodontist Appointment

My 10yr son had an orthodontist appointment yesterday.  We prepared him with a social story which worked really well.  He defo needs a brace as could loose teeth if the crossover bite is not sorted soon. 

When the dentist went to do the impressions my son was fine until the mould with putty in was put in his mouth. He couldn't tolerate the texture or the taste and ended up being sick. 

I would like you know if anyone has any ideas on how we get through this so he can have the impressions done.  If anyone has had similar experience.  Any thing advice / guidence would be greatly appreciated. 

Thanks in advance :)