"To need" in Spanish is necesitar, which is a verb that ends in -ar. This verb conjugates like so:
It doesn't matter that the subject is male; if you're using the usted form of the verb, it must be necesita.
As for ayuda, that's a noun. She needs a ticket (Ella necesita un boleto), he needs a bag (El necesita una bolsa). Nouns retain their gender even if it's for the person of opposite gender.
Think of it this way; let's say a girl has a tomcat. La nina tiene un gato, even though the girl is female. It doesn't change the gender of the cat.
Please excuse my lack of accents.
"Mister" is not incorrect, but "sir" is preferred because "sir" is unambiguous and more polite. "Mister" begs a surname or affirms an occupation, e.g., if you respect the gentleman or the man who mists so much, then why have you forgotten his name? "Sir" is simply a title of respect, and indicates the speaker's willingness to be cordial and/or deferential, without implying any prior introduction. It elevates both parties - one by honor, and the other by their devotion to honor.
If mister is used with sincerity, there is nothing wrong with it, but it's out dated. On the USA west coast we address a stranger or a superior as sir. In some cases, mister is used in a disrespectful way, like "look here, mister, i dont like your tone" or "i think you need to step back, mister bossy pants". If a man were to say something i strongly disagree with, and i wanted to imply he is crazy, i might say "mister, do you need help?" But it can also be used as an endearment, like we might call a boy or man who is dressed nice Mr. Man or Mr. Handsome. Its also used towards pets or children, like "mister munch a bunch" or "sir barks a lot".
There's so much wrong with this.
You're not redefining mister, just because you use it ironically. Just like no one would care if you called cold things "hot", to consider that it meant cold. We know what sarcasm/irony is, and it is a phenomena that doesn't redefine words, but actually requires them to have the definition they do.
It wouldn't be offensive to use mister ironically if mister weren't already a term that meant respect.
As another example: Genius - a person who is much smarter than average
This word is also used ironically to call people stupid, such as "Who could have predicted that one genius?" Which is an ironic phrase meaning "Obviously, moron."
Sir can be used ironically as well. Also, using sir to address strangers could even be seen as offensive to those who earned the title "sir" through knighthood.
As I've understood it, and according to several other Q&A-sections the 'Sir' implies a formal situation in which case we use third person (El/Ella/Usted) for our verbs even though it's really a second person situation (Tu). This is called formal second person and is in the case of this example singular.
Yo necesito Tu necesitas El/Ella/USTED necesita
Similar to many other european languages. I suppose a likeness could be drawn to how we in English could ask a royal person 'would HIS highness require a ride' VS 'would YOU like a ride'.
Sir is not a shortened form of Mister.
Sir and Mr. are just different honorifics, though sometimes they are used the same.
Sir can be used as a polite way to address a stranger, or for it's more direct meaning as a title of knighthood.
Mister can also be used as a polite way to address a stranger.
Necesitas is informal, like if you're speaking to your friends, family members and people with whom you are on a first-name basis.
Usted necesita is formal and you would use it to show respect to someone you don't personally know, like your boss, an old lady in the street, your teacher, a receptionist at the hotel etc. :)
It is not that this is plural or singular. Both of your examples are singular. It is a difference of formal vs informal. Whenever you use only a first name it is generally informal. In contrast, if you use last names and titles it will be formal.
Señor, usted necesita ayuda? (formal version)
Migual, (tú) necesitas ayuda? (informal version)
Both are still singular though. The plural 'you' would use 'ustedes'.
Señores, ustedes necesitan ayuda? = Gentlemen, do you need help?
Miguel y Ana, ustedes necesitan ayuda? = Miguel and Ana, do you need help?
I think what is confusing you is that the 2nd person singular form of the verb always ends in 's'... necesitas. Despite this it is not plural.