Of course, rather than expose him to 8088 assembler,
I?d recommend
you run right out and grab a used TRS-80 Color Computer - cheap,
and 6809 assembly is very very nice - no segment registers.
The downside of *that* is that if he ever decides to
use the ?Sign Extend?
instruction, you?ll have to have a talk with him about the birds, the bees,
and where little subroutines come from :-).
I suspect you'll need some 'support' when you come to unconditional
relative branches too :-)
-tony