I guess if you really have to, you could put it off...though I don't like that idea...
Just tell him that eventually he's gonna NEED that ride cymbal. He'll want a certain sound that only the either only the bell of a ride can produce, or a sound that tapping the top of a crash cymbal just can't produce. Crash cymbals suck for riding on -- that's why there are ride cymbals.
And if he's a REAL heavy hitter, look into heavy cymbals, too. IIRC, the HHX Series s pretty thick, but I'm really not very familiar with the Sabian line. Zildjian Z Customs is what I would get him. They're made for the heave hitters.
Hey, here's another thought. Get an 18" or 19" crash. Since he's a heavy hitter, he might like the sound of that bigger cymbal. And hey, it's a good start to get used to the feel of a ride cymbal. Granted, it won't sound like a ride cymbal, but it might feel like one. Anyways, it's just an idea, and IMO, it's a better choice to have the 16" and 18"/19" crashes rather than the 14" and 16".
Sorry for the rambling, I hope it makes sense. I have a tendency to ramble, and it'll make no sense...
Good luck!
- Justin -