Sort by an expression:
=CDate(Fields!Month.Value)
This works OK for data in your format, even if it's a string. The expression converts the string to a date which can be sorted correctly,
I have some sample string-based data:
select Month = '2013 October'
union all select Month = '2013 November'
union all select Month = '2013 December'
A simple table based on this:
Sort by [Month]
:
Sort by =CDate(Fields!Month.Value)
:
As per the blog post you include, you could also easily move the date conversion to a calculated field and sort by that, which could be useful if you're using it in multiple places.