The OTD subject is quite complex, and the efforts to shoot simple explanations or general advice are, perhaps well intended, lame efforts. It is presumptuous and more often false to point a finger at Internet and shmiras einayim issues. The rush to find his Rov is frankly ridiculous, as it is strange to expect that he actually has one that can impact him. So let’s try some generalizations that are actually valid, and seek avenues for remedy.
First question. When we say “Off the Derech”, we must then offer a definition that identifies the issue and is broad enough to include most who are OTD. What is the “Derech”? Until we answer that, we only know the individual is leading his life in a different direction. Maybe he is making more sense than we are, and the problem is us. And we should not forget that our “Derech” includes an awful lot of frank hypocrisy.
Second question. We each have a philosophy in life, and it guides us in making choices, especially those about lifestyle and beliefs. What is his? How does it differ from ours?
Third question. What do we know about experiences in his life that contributed to his making such choices?
What we do know is that he has chosen to abandon the “Derech” in which he was raised. For some reason, it has been experienced as painful enough to him that escape was his preferred defense. Trauma? Systemic hypocrisy? No, it is unlikely that he is seeking to run into the welcoming arms of secularism or free expression of taavos. He is running away. What is he running from?
It is a sad generalization that an overwhelming percentage of our rabbonim are ill equipped to handle such issues. A erudite pilpul based on Chovas Halevavos would be interesting to many of us, but likely to bore him. Few rabbonim have experience in connecting with someone shut down.
Lastly, many of us, particularly the best intentioned rabbonim, would jump to intellectual arguments to convince him that he has made bad choices in “going OTD”. Such efforts are a total waste. It is an exception that such escape occurs for logical reasons (as noted by an earlier commenter). Nearly every single situation involves the emotion, not the intellect. The Rov experienced in this might be effective. The best magid shiur, Rosh Yeshiva, pulpit Rov, etc. is unlikely to have such skills and talents, as they are less often needed for their positions.
Someone mentioned Reb Elya Brudny. He is one that has a huge heart and incredible sensitivity besides amazing insight. He is also inundated with tzoras Yisroel, and accessing him, and others in similar positions may be difficult. There are others that share Rav Brudny’s maalos, but are busy with Klal needs. And much is needed to provide the guidance for the trip back to the “Derech”. And the individual needs to want to return. And until there is desire, all efforts are futile.