They seem like a great idea at first, but when you consider how many oranges you need just to make a full glass of juice, you'd have to buy a ridiculous amount of fruit to use the thing on a regular basis. Factor in clean up, and they're a waste of money IMO. Drinking juice won't keep anyone from becoming fat or make anyone that's already fat become skinny. If you're looking out for him, buy them a gym membership.
You need about 6 oranges to make a glass of juice. Fresh juice is much healthier than anything from the store.