for what its worth-
we lace rims dailey-
and a lot of OEM rear wheels have inside /outside spokes of different lenghts.
certainly the Honda CR and CRF line up..
(your sons CRF250R has different length rear spokes,, the front spokes however are same lenght )
so to anyone ,before lacing or ordering spokes be sure and check..
its fairly common practice to have different lenghts inside verse outside.
*shrugs*