Here’s a contrarian viewpoint for your own protection.
First, emails are not formal, so you have not interrupted a formal communication. Why then elevate it to formal and so make yourself look naïve?
That said, and taking you at your word that he engenders fear, the only gentle way to inform him is not to actually inform him at all. Believe me, he is not interested in your dilemma, nor in noble or fawning words.
One approach is to use a go-between, such as an executive secretary if he has one. Without mentioning your surname, simply hand it to his secretary with the simple statement that this came to you by mistake. An administrative aide’s job is to know what preens and what ruffles a boss’s feathers, and to act accordingly.
If no one is available to bring the electronic mishap and misdelivered letter to the gent’s attention, simply forward it back. It is likely that your own address was but one of many addressees, so word may have already spread.
The problem with making excuses to him, aside from wasting two persons’ time, is that you expose yourself unnecessarily. Do you think he will take any kind of contrived politeness as other than self-promotion? I wouldn’t. Neither would you.
Finally, seniors do not typically use email, so take advantage of the protective layer that surrounds any exec and forward said mail without further delay.
Or shred it and throw its tattered remains in the trash.