Let me try to answer you questions.

“(a) I do not know how someone who got along so well with me could abruptly pull away for no apparent reason”

Actually there are plenty of reasons you can imagine. As Peace stated in the post above, he may have got problems with his wife. Now, he may actually talk with you about those problems. However, that will promote a complicity between you and him at the expense of the relationship with his wife. Hence, that man would risk to make the relationship with you stronger and weaken the relationship with his wife. No matter what logical argument he may propose, you are always going for a way to talk and meet him, and he may actually like the idea. He may have started having feelings for you, but he is trying to suppress them for the sake of the marriage. Of course these are not things he can discuss with you. If you care about your friendship, you can wait for the day the problems with his wife are solved or his feelings have abated.

“(b) I am tempted to keep on trying to reach out to him assuming he will get back to his normal self. ”

To become and to change, to morph in a new self should be his decision. He’s the owner of his soul. As stated above he may not be willing to discuss his reasons with you, he may not even be able to verbalize his reasons to you. When the feelings push you from one side to another depending on where you are, it’s quite hard to think straight. One needs time to remake oneself. Do you care for him, do you love him as friend? You have no ownership of him, no commitment with him, accept him even in his change. One day he may be able to discuss his reason with you.

“If it is time to let go, why am I not able to? Is it fair that I have to let go of the only bright light in my life?”

You do not have to let him go. The memories, the words, the support, the care he showed, the friendship, the inspiration you had are real and yours forever. He may be changing and dieing to your eyes, but the old man he was is still there in your heart and still lives somewhere inside him. Repeat to yourself what he had taught you, be proud of what you have done and do not disappoint him. Wouldn’t you like to show him your successes when he is out of what he is passing through and able to discuss once again with you? Even if he changed and died forever, wouldn’t you honor his memory with your achievement?
If these thoughts are not enough for you to stay at peace, it may be the case you are really in love with him. And that may actually be the reason why he does not talk with you. He may be waiting you are back to your old self. If that is the case, let him go as lover and he’ll be back as friend.