Well, not being there to observe the bird is hard to say what the reason for his no longer talking might be. Usually, when a bird stops vocalizing, there is a reason for it BUT, on the other hand, birds do stop using words or phrases they no longer hear - the only thing is that, normally, they would substitute the 'old' words for 'new' ones. Let me give you two examples: Mami Zon loves to sing in the morning but she did not do it for over a week recently because my daughter and her kids came to visit so it took her all those days of seeing them for her to feel comfortable singing. Then you have Sophie CAG which had lived in a home where the husband was abusive and she used to say heartbreaking things like: Ooooooh, God help us! Oooooh, Lord saves us! Stop! Stop! You are making it worse! but she no longer does. At all! She now says things like Give me a kiss and tells Linus Too to 'Shut up' when he screams

Funny thing is, I tell the dogs to shut up all the time but I've never done it to a bird so she actually figured it out on her own.
What intrigued me about your posting was the comment you made about you not saying Hello or Pretty Bird to him. What do you say when you first uncover his cage in the morning? Or when you praise him? Because, to be honest, they are such super common expressions people use with parrots that I never considered somebody not using these words to them.
A bird that is not 'talked to' will not talk and that is pretty much the rule of thumb. But, in order for a bird to repeat a word, it needs to hear it. Some people (like my husband, for example) do not realize that animals don't go by the actual meaning of the words but by the sound of it which they've learned means something specific. Let me explain. A dog (or a parrot) will learn, thorugh repetition, what a certain sound means and will react accordingly to it - with the added bonus that a parrot will, if taught correctly, actually learn to use the word to express itself. My husband talks to the animals all the time but he talks to them as if they were children and could comprehend different ways of saying the same thing instead of repeating the word or phrase they do know (so they never do what he wants them to and complains about it). So, while I a dog will learn that 'Fetch it' means he is to go get the toy that's being thrown, a parrot will not only learn that 'Go Home' means he needs to go back into his cage but also that he can use words to ask for something or express a feeling (as in a parrot that loves you and repeats the often heard 'Give me a kiss' and kiss you right after). This is important to parrots because they are highly social animals that actually have a language (a study on one single species of South American parakeet determined that species had over 500 words or phrases they use to communicate among themselves). An example: some time ago, I was having work done in the house and moved the cages. One night, I forgot to give Sweetpea and Zoey Senegal their dinner so, when I came downstairs after feeding the big birds, Sweetpea was screaming: 'What's the matter? Nite nite eat it!" He was asking for his dinner (he actually made up the nite nite eat it to mean dinner because, to him, 'eat it' means food). So, when we teach them words they can use cognitively, we are actually providing an enrichment to their lives and a way to give them a flock 'feeling' because they are intellectually interacting with us and communicating on the same level - same as they would do with their own peers.
Again, I cannot tell you why the bird stopped using his words but one probable reason is that he is not hearing them from you so he figured you have no interest in talking with him. What I would recommend is for you to start using the same words/phrases with him all the time - like, in the morning, when you uncover his cage or when you come back into the house after being absent, say Hello to him. I'll tell you the words phrases I commonly use with mine (which they ALL understand and some say back to me). When I first open the blinds, I say Good morning. When I open the cage for them to come out. I say Come out? Que rica papa! And Eat it! is for when I serve them food. Go home means go back to your cage. No is self explanatory but it is usually followed by a Don't touch (when they are trying to get something they shouldn't) or a Be nice! when they are squabbling with another bird (they even understand the "I am watching you!"). Then you have Pretty Bird, Sweet Baby Bird, I love you, Peanuts (all nuts are peanuts), Whachoodooing? Give me a kiss - immediately followed by Que rico beso (what a nice kiss), Naughty bird, Bad bird, etc etc and, at night, there is the 'Nite Nite, sweet dreams, I love you!' My birds understand lots and lots of phrases and words and obey almost always my commands without me giving them a single treat for reward. And I think it's because I am so consistent and persistent BUT I also believe their learning what the words mean enriches their life as it must give them a sense of belonging and acceptance into the 'flock'. It might not be what nature exactly meant for them but it is as close as I can make it.
So, do talk to your bird using always the same words to indicate the same action. He might or might not learn to repeat them but he will understand them and appreciate it greatly.