I can totally see why he would stress about it. I have no advice other than go to your GP as he might be able to refer your Ds to a cousellor. You dont even need to take your Ds just book the appointment in his name.
There is a thread for parents of anxious children. I will bump it for you as someone on there might have some more ideas for you.
My ds goes to High school this year, obviously this brings numerous natural worries. One in particular is his fear that when the fire alarm goes off (which it will even if just for drills) he will wet his pants. His fears arent unjustified as this happened more than once at primary school and as late as primary 5. Nothing i say seems to help, he is adamant it will happen and he will be bullied mercilessly for it. Short of following him into public buildings and secretly setting off fire alarms I am at a loss as what to do can you help me?