I have been re-reading Harry Potter and the Deathly Hallows and there is a point which I don't understand about Ron's character. He has been very reluctant to use Voldemort's name till Half-blood Prince and it has been very clear in the books
Harry pulled back his bangs to show the lightning scar. Ron stared. "So that's where You-Know-Who "Yes," said Harry, "but I can't remember it." "... and until Hagrid told me, I didn't know anything about be ing a wizard or about my parents or Voldemort" Ron gasped. "What?" said Harry. "You said You-Know-Who's name!" said Ron, sounding both shocked and impressed. "I'd have thought you, of all people --"
Book 1: Ch 6: The journey from platform nine and three-quarters
"Harry Potter speaks not of his triumph over He-Who- Must-Not-Be-Named -"
"Voldemort?" said Harry. Dobby clapped his hands over his bat ears and moaned, "Ah, speak not the name, sir! Speak not the name!" "Sorry" said Harry quickly. "I know lots of people don't like it. My friend Ron -"
Book 2: Ch 2: Dobby's Warning
"Malfoy's dad must have told him," said Harry, ignoring Ron. "He was right in Voldemort's inner circle --"
"Say You-Know-Who, will you?" interjected Ron angrily.
Book 3: Ch 11: The Firebolt
My scar hurts, and three days later the Death Eaters are on the march, and Voldemort's sign's up in the sky again." "Don't - say - his - name!" Ron hissed through gritted teeth. "And remember what Professor Trelawney said?" Harry went on, ignoring Ron.
Book 4: Ch 10: Mayhem at the Ministry
"Wow, scary thought, the boy You-Know-Who," said Ron quietly,
Book 6: Ch 14: Felix Felicis
Ron grew up in the wizarding world having heard stories of Voldemort's terror. Moreover, everyone in his family, including his parents didn't say Voldemort's name so his apprehension towards the name made total sense.
"And it hasn't been seen for thirteen years," said Mr. Weasley quietly. "Of course people panicked. . . it was almost like seeing You-Know-Who back again."
"It's what You-Know-Who's supporters called themselves," said Bill. "But what were Voldemort's supporters -" Harry began. Everybody flinched - like most of the wizarding world, the Weasleys always avoided saying Voldemort's name.
Book 4: Ch 9: The Dark Mark
Even Hermione who was muggle born but having read all about Voldemort's reign of terror refrained from using the name up until Order of the Phoneix after which she started saying Voldemort's name.
ʹHarry,ʹ she said timidly, ʹdonʹt you see? This... this is exactly why we need you... we need to know what itʹs r‐really like... facing him... facing V‐ Voldemort.ʹ It was the first time she had ever said Voldemortʹs name and it was this, more than anything else, that calmed Harry.
Book 5: Ch 15: The Hogwarts High Inquisitor
But then in Deathly Hallows Ron is suddenly OK with saying Voldemort's name in normal conversations and even using his name casually in jokes.
“Oh, of course,” said Ron, clapping a hand to his forehead. “I forgot we’ll be hunting down Voldemort in a mobile library.”
Book 7: Ch 6: The Ghoul in Pajamas .
“Why didn’t Voldemort declare himself Minister of Magic?” asked Ron.
Book 7: Ch 11: The Bribe .
“But according to you,” said Ron, “Voldemort’s got Ollivander locked up somewhere. If he’s already got a wandmaker, what does he need another one for?”
Book 7: Ch 12: Magic is Might
Once they are back from Ministry Ron is suddenly scared to use the name again.
“I’m sorry,” Ron said, moaning a little as he raised himself to look at them, “but it feels like a—a jinx or something. Can’t we call him You-Know-Who— please?” “Dumbledore said fear of a name—” began Harry. “In case you hadn’t noticed, mate, calling You-Know-Who by his name didn’t do Dumbledore much good in the end,” Ron snapped back. “Just—just show You-Know-Who some respect, will you?”
Book 7: Ch 14: The Thief
Did he just start using the name because now both his best friends has been using it for some time now? If so then why stop again as they did not know anything about the name being a taboo till this point.