If you were with your autistic son and he saw there was a fire and said come with me so I can pull the fire alarm what would you say? If there was glass and your son started to kick it and he missed it on the first try would you help him to posistion himself to get a better kick or tell him to kick back and up?When he is pulling the handle down how would you let him know that he is doing a great job so he knows the handle will work?What if other people were around would you say something if they asked why was your son pulling the alarm? How would you tell the people that its not a prank etc? If someone tried to push your son's hand away from the handle how would you get them to let him pull the handle?
Why don't you just weld some pipes on the existing exhaust? They have pre-bent pipe fittings and stuff at your local auto shop.
This is sounding very expensive! Leather curtains? You don't need to spend that kind of money to get relief. The J.C.Penney catalog has thermal backed curtains that are designed for just the purpose you outlined.
