If you Google "Prince William Ring" you will see that Prince William will not wear a wedding ring after his marriage. For example, see this ABC story.
Rav Moshe Feinstein has a teshuva about 2-ring ceremonies in Igros Moshe Even Ha'ezer III 18. (See here for a copy of the teshuva.) Briefly, he says that either it's chukas ha'akum, and if it's not, it's asur because you're changing things and it will lead c"v to forgetting halachos.
I am not aware if Rav Moshe discussed if a man may wear a wedding ring if it was not given at the ceremony. My feeling is that it's not asur. In certain situations it would probably be beneficial for a man to wear a wedding ring, depending the types of people he interacts with. For example, a man in kollel probably has no reason to wear one. If a man is married and spends a lot of time on a college campus - whether as a student or teacher - I think it would be a good idea to wear one (though there is absolutely no obligation to do so).
Once I'm mentioning an Igros Moshe, I saw a teshuva over shabbos that really got my attention: YD III 91: he didn't want someone to translate part of his Teshuvos into English.
This is undated and was written to his grandson R' Shabsi [Rappaport]. I suspect, but have absolutely no specific facts, that this was in regard to a particular book. Why do I say this? Because Rabbi Dr. Moshe Tendler published a book of Rav Moshe's Teshuvos in English. Also, he wrote a Teshuva (OC IV 74) on common questions in Hilchos Shabbos to Rabbi Shimon Eider, which were probably included in Rabbi Eider's English book on Hilchos Shabbos which have a haskama from Rav Moshe (and he did publish the teshuvos in Hebrew in the back). There are probably many other examples like this. That's why I assume he was writing about a particular book.