BASIC really isn't useful for anything.
It might be better teaching him another interpreted language that has real-world use, like Perl, Python or Ruby.
All of these are available on IRIX. In fact, IRIX ships with Perl (old version, but there anyway).
hmm - I bet the whole generation of programmers brought up in the 80's on Spectrums and BBC's and AppleII would disagree strongly. This core of BASIC programmers has gone on to do a lot.
With a BASIC in ROM you could use the machine out of the box, sure it was limited but BASIC is forgiving and very easy to learn.
BASIC is a useful, easy to learn (and remember) language....
Being one of the '80 generation, you bet right.
Perl is just a way to learn how NOT to program.
BASIC is plain, simple and fun. Children want to see/hear things asap and a BASIC with gfx/audio commands is strongly recommended.
Forget OOP: not for children.
And I would rather choose COBOL/FORTRAN than Perl/Python/Ruby as the first language to learn.
My personal suggestion:
Try with BASIC (with gfx/sound), then if your son is willing to know more, go directly to Assembly to check for real passion.
Great coders perfectly know (but seldom use) Assembly language.