The man who I bought Sherbie from told me that he is a male, but now I'm starting to see white rings around his nostrils and I've heard this means that he is a she?

It could do, if the cere is a pink and smooth looking then that would mean male, if the cere is a pale blue or turning paler all over then you might have a female. Is there any chance of a picture that would help us tell you plus we love seeing everones birdie. If you got 'him' from the breeder they are normally good at telling the gender of babies, but of cuase are not always correct.

yep we love pictures, Pretty has rings around his nostrils, but he is a boy because of everything Nerwen mentioned above.

It might be that his cere has started to change colour. For instance, my Arkady has a pink cere but around his nostrils it is starting to go blue, which gives the appearance of white rings. When I first got him his entire cere was pink.