i've just ordered a new iPad 3 for my sons Christmas present (don't tell him!! )
Too late. I just spilled the beans!
Your son can sync 20 devices to the same iTunes library - that is if you are generous enough to buy him 18 more - without any worries at all. I sync 3 devices to my iTunes library and I select the exact content that I want to put on each device by setting the sync presences for each device in iTunes.
You can read this for more information on how the sync preferences can be set up for each device.
and this one as well.