That's certainly possible. Just set up a Google voice number for him and download the Google voice app from the market. You can text over wifi for free. That's how I text anyways on my Droid 1, since it's free and I don't have to pay extra money on top of the unlimited data each month for texting.
Thanks Bharrismac. I'll give it a try when I get a new Droid. I'm just not sure if I should wait for the Bionic or not since we don't have 4G in our area yet but supposedly this year. My MotoDroid is really starting to lag.