Well, he's got everything else, and more or less is ready for Wind Waker... Plus, since he knows about stuff from Wind Waker, it's the best to get into first.
And another thing; since Majora's Mask is the direct sequel to OoT, although I'd recommend Majora's Mask in a heartbeat, it
does make a little more sense to play through OoT to make sense of what the heck is going on with the intro...
"Who's that friend Link's looking for? Why does Link have a horse? What's that instrument that weird guy took, and why's it so important? ... How'd he get that instrument anyway?"
If all those weren't a factor, I'd definitely say 'go for it', ASAP.
