I have a website called JuicingJuice. Its link is http://www.juicingjuice.com if you would like to check it out.
Anyway, I have been juicing with my kids for over a year now and playing around with lots of different recipes. Most often, I just throw together whatever I have in my fridge.
What I have just recently discovered is that to make the juices a million times more palatable than they have been, I do two things to every juice I make now.
No matter what, I add about 1/2 an inch, to 1 inch of lemon. This includes the peel since it has most of the flavor (this is the only citrus fruit that you can eat the peel and have it taste good) Adding the lemon changes the whole flavor of the juice and makes it almost taste like lemonade, no matter whatever else you throw into it.
The other thing I do is shake it in a shaker of ice before drinking. If its cold, it is sooo much better. When I first started juicing I'd drink it right out of the juicer. You wouldn't believe how many times I almost gave up halfway through finishing it. Now I LOVE to drink them. In fact, I usually put the juice in a fancy glass and sip it to make it last, its that good!
One other thing I meant to mention, I always add one apple to any combination that doesn't have another source of sweetness. Vegetables are surprisingly sweet when juiced, so only one apple is needed. That along with the lemon and it really is delicious!
My latest favorite juice I have on my facebook fanpage (JuicingJuice) and my original Juice Juicing. I regularly post new recipes on there as I come up with them. I also have lots of recipes on the website (but I'm still adding to that and many of them do not mention the lemon or the ice because I wrote them before discovering that, heehee)
1 wedge of cabbage
3 carrots (large ones)
large handful of spinach
2 or 3 stalks of celery
1 inch of lemon (organic, non-waxed)
Really though anything you have on hand will work. Its all about trial and error, though most of them will be pretty good no matter what you throw in.
Good luck I hope your son finds juicing fun and will give it a try. I'd love to hear how it goes.