With a name like that, he is a native speaker. That is an old fashioned way of singing that is not often heard.
But in Scotland and some other places, the Rs are rolling, like in this song - otherwise most native English speakers have a hard time with rolling Rs.
If you listen to very old speeches by upper class people in Britain from early in the last century, then they sometimes speak in the way that McCormack is singing. I don't know why though, and nobody ever speaks like that anymore - well maybe the Queen, to some degree.

I agree with Lampada that songs can help learning a language. It certainly helped my English.
When I was a kid I memorised some Russian songs, in school and in choir; we just learnt the words without knowing the exact meaning, so we could sing the refain in Russian, or something like that. These words have been a lot easier to remember afterwards. But it was not a lot of songs, maybe 5 - 10 so it did not give me a large vocabulary.

I also listened a lot to Russian radio for a while, and some hits were played again and again and the words get stuck in your mind, even if you do not know what they mean - then when you come across them in study, they are easier to remember.