I think, first off, his falling in love with you is a testament to how well you have been able to create the illusion and the level of your skill. So, kudos.
But, you are a compassionate lady who now wants to find the best way out with the least hurt. So, I put myself in that gentleman's shoes, and I think that the advice is that which Cherise gave is probably the gentlest way to go. Indirect, but given some time to think on it, he might get the message. Or he might not (I've been oblivious to indirect hints like that).
It is a question of how you have that talk with him, and there isn't any one right way, as we are all individuals and cookie cutter approaches frequently don't work. But, you know that as you are able to read each of your gentlemen and adjust to their personality, quirks and preferences. Having read your comments on this board, I have no doubt you will find the right way to handle it gently and with compassion. Hurt and anguish is highly likely, but better soon rather than later. Is it possible for him to continue to see you without it also continuing to add fuel to his affections and feelings? I don't know that it is, though you do say he is a truly exceptional individual.
Do you know of any other ladies you can introduce him to? Maybe helping him find someone else to fall in love with would be the most compassionate thing (though not necessarily the easiest). From what you have said, this sounds like what he needs. He is widowed, and he needs someone else to fill that void, to be the object of his love. If you can't be that person, then helping him (or coaching him) on finding that lady might be what he needs. All the best on crafting the most compassionate approach.