Alright, I have saved the first maiden in both single and multiplayer.
Single player is VERY fun. I'd say it's worth it to buy just for single player, although it does get a bit irritating having to switch Link formations and when there are four seperate switches you have to do alot of button pressing to control each Link individually, which gets a bit tedious. Not a big deal though, since most of the time your companions will just follow you.
Multiplayer, though, is much better. It's simply a blast, some of the most fun I've had in a while. Even with just two players it's fuve times more fuin than single player. It's be perfect to play with your son because you have to work together and communicate alot, while there's still the competition to see who can get the most Force Gems and who can kill the most baddies to come out on top in the end (there are rankings at the end to see who did better.)
So it would be worth it to pick up another gameboy. Plus if you get another gameboy it opens up the possibility of playing your son in multiplayer GBA games.