While it is odd that a twelve year old boy is using words such as "account" (despite his upbringing) in such a manor it's doesn't seem grammatically correct (I think he should have said "on the account") it does give a nice realism in that it's not right and a feel that that is how Harry is speaking.
'On account of' is a fairly common idiom, at least in some dialects -- I tend to think of it as chiefly British, but evidently not (http://www.bartleby.com/68/69/4269.html), and if it were as standard in Australian as in American English you'd probably be accustomed to it. It's not weird for Harry to be using it, but it is informative: an adult wouldn't use it in a formal context, nor very possibly would someone the kids' age.
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'On account of' is a fairly common idiom, at least in some dialects -- I tend to think of it as chiefly British, but evidently not (http://www.bartleby.com/68/69/4269.html), and if it were as standard in Australian as in American English you'd probably be accustomed to it. It's not weird for Harry to be using it, but it is informative: an adult wouldn't use it in a formal context, nor very possibly would someone the kids' age.