Definetly get a Garmin as you can sent cache co-ordinates direct to a Garmin from the geocaching.com site.
I have a Garmin etrex Legend HcX. I think its very good. You can get ones with more features if you pay more but it depends if he would use the features or not. I've heard poor reviews of the Garmin Colorado which is their top of the range one - too many teething problems at the minute. The Oregon is meant to be good but pricey.