The roots of the Sunni/Shia divide stem from pretty much the minute Muhammad (PBUH) died. It was based on who they supported as his successor. Sunnis believe it was Abu Bakr, who was a friend of the Prophet and his, like, chief adviser. Shia Muslims believe that it was the Prophet's cousin/son-in-law, Ali, who should have been the successor. The Sunnis won out, though Ali did become the fourth caliph. His son's murder really kinda cemented the divide. To this day, Sunnis make up about 80% of Muslims. Another big difference is that Sunnis tend to stick to the 'sunna', which are the teachings of the Prophet, while Shi'ites see their Ayatollahs as, like... Popes, a reflection of God on Earth. God's vessel. Many Sunnis thinks Shi'ites are heretical, many Shi'ites see Sunnis as so focused on purity, they've given rise to the asshats of Wahhabism. (Groups like ISIS tend to be majority Sunni, for example. And like to kill Shi'ites, whom they see as not Muslim.) Saudi Arabia is mostly Sunni. Iran and Iraq are majority Shi'ite.

And, of course, Shi'ism has divides within itself based on which of the great imams will return from hiding (for lack of a better word) and fulfill God's divine Will. Most Shia believe it's the twelfth imam and, hence, are called Twelver Shia, but the Fatimids (who were fighting the Abbasids for control in Saudi Arabia at the time) tended to be Ismailis, who are Sevener Shia. (There are also Fiver Shia, but they're a small sect.)

If you've chosen to make him a Sunni, I sincerely, sincerely doubt he'd change over to Shi'ism. While there's never been the kind of fighting between them that we saw with Catholics and Protestants (even unto today), they are very much divided. If you were looking for a way for him to get bored and go through a period of exploration, may I suggest the Sufis? They're mystics. If you've ever seen the Whirling Dervishes, they are a branch of Sufism founded by the poet Rumi. They're the Mevlevi Order and were common in the Ottoman Empire, though founded before its rise (in the early 1300s; the Ottoman's conquered Constantinople in 1453).
-Quaddy

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