Why is my senior dog suddenly afraid of thunder?

We have a 13 year old dog who has never had any issues with storms but for the past 6 months he has suddenly become very scared when it starts thundering outside. He has literally destroyed the carpet by the doors...the door to go outside, the door to our daughter's room, and the door to the bathroom. I have no idea what to do about this as it always seems to storm while we are at work so giving him a calming treat or something similar isn't really an option (I have tried these and they don't seem to have any effect on him anyway). If someone is home he doesn't even flinch when it thunders but if nobody is home he tears up the carpet every time. Suggestions?