Epoh - a couple of weeks ago we were at the Science Center and saw their film about ways to reduce your carbon footprint. Could something like this help your ds? I mean give him a tangible way to make a difference about even such a big issue? For example DD made me immediately unplug our electric can opener when we got home. Maybe he could figure out ways for you to drive X fewer miles each week, what would happen if you changed the temperature in your home by X degrees, calculate the savings if you dried X loads of laundry outside instead of in a clothes dryer, etc.
If he is going to think (or even fixate) about this problem maybe you can change his thought process so he looks at ways to help rather than just focus on the scary part. Or maybe take a page from JonLaw's post and have the family spend a day or a weekend roughing it - see what you would in fact do if you really had to? I can totally understand that the "don't worry" approach won't work - with mine that would just send her into overdrive. If I am not going to take it seriously then she will have to worry twice as much to make up for it.
Just a few ideas. Not sure if they help but thought I would throw them out there. Good luck!
