There is no REAL reason why they use the word 'boii', but the two i's (ii) seem to be pretty popular among the chavs in the 14-18 age group.
There are other words such as
iino (i know)
Whats worse, is that some people use the word 'boi', Message to the people who say 'boi', this doesnt make you one of those lazy typers, as its the SAME amount of words as 'boy', it just makes you look stupid.
Meaning: That boy was attractive
David: Yo Mike you're an ugly fuck!
Ben: Boii !
Yvonne: OMG we have a snow day tomorrow! Boii !