divvy198509 saidIsn't it supposed to be 'If I WERE your boyfriend' ?
This. The subjunctive is such an abused part of grammar.
Eh. A great many linguists would disagree. There's a huge difference between spoken English (which changes frequently) and written English (which changes very little).
Momentarily derailing the thread...
Most prescriptive grammar textbooks etc. are concerned with written English, which is infinitely more easy to teach and deconstruct, since most English-speaking countries use similar standards (as demanded by print; see early printed books for an illustration of what happens when everyone spells things differently; it was a mess). But, there's a reason there are different English styles of writing (like Chicago or AP; various dictionaries; etc.)--English has no language regulation to set universal spelling or grammar standards, unlike the French or Germans (http://en.wikipedia.org/wiki/List_of_language_regulators
However, writing is less prone to change because the words are fixed and physically captured. Not so with spoken English, which has an abundance of unique and colorful dialects (several of which are endangered of becoming extinct) that have their own subsets of grammar, none of which are "incorrect" in the context within which they are spoken.
I believe in either dialect, it is perfectly acceptable to use "was" instead of "were". The same is likely true in Appalachian English, most of the varieties of at least the Atlantic maritime American dialects (like NC's Ocracoke or MD's Smith Island), and so on.
Given that Rap/R&B/etc. are derivatives of AAVE, it follows that they'd be generally influenced by the dialect's grammar as well.
... Derailing over.
That said, I'm not a fan of Beiber's lyrics, mostly because I don't really identify with the intended demographic. The actual music's decent.