Uh-oh, fear of an audit is what makes me keep every record I have and why I pay $$$ for a CPA. No way would I go myself. I pay for my CPA's expert advice and handling and its his professional responsibility to make sure everything is in order. Since he created the return it is also his responsibility to defend it. I'd fax him the letter from the IRS with a note that I expected him to defend the return in question on my behalf, call the IRS and tell them my CPA would be handling it then monitor from a distance by staying in contact with both the CPA and IRS separately.
PS, one of my CPA's underlings made a huge mistake with my local filing once, to the tune of $1,000! I let the CPA know what was the problem and he stepped up and paid the difference as soon as he looked into it and saw the mistake was on his side. If this is really all that simple the CPA shouldn't charge you a dime. It is HIS work being called into question, not you.
Good luck and relax. Having a CPA pretty much means you are in the clear. Just make sure you only give him copies to look at, not originals. Keep all your originals in a safe place so you can prove you did what you were supposed to do in case there really was a gross error on your CPA's part so it doesn't land in your lap.