I think you answered your own question when you asked where the original gramps went. As you mentioned he passed away. :)

When the author refers to “in the name of thy son” he is referring to what is said at the end of a talk, not a prayer. I’ve often noticed that people mistakenly do that as well at the end of their talks. In a prayer it’s fine. I agree with the author’s advice about being patient with others and not being overzealous to correct others who are still learning. When we feel to correct others, we should do so with kindness and ensure that love for them is our true motive (D&C 121:41-44).

Mark,
I think you may be slightly confused here. If you look at what Gramps said, you’ll find that he said that it concerns him when “people close a talk or a testimony” with “in the name of Thy son”. When someone prays and ends the prayer this way, it is perfectly appropriate, just as you said. When someone gives a talk and ends it this way, the speaker either believes that someone in the congregation is the parent of our Savior or is not thinking about the meaning of the words. That is why Gramps is concerned. I hope this clears up the matter!