Is it productive to use a Stylus with the iPad?
The Cosmonaut is about the only stylus worth the effort in my opinion. I've tried a few different ones, but the Cosmo' has always felt right. I think it's to do with it's weighting and general sturdy feel.

I think with practice your son will produce excellent results using the Cosmonaut, especially with the right app (Paper or Sketchbook Pro are excellent for art.)

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'Random' 2yr update: After using the Cosmonaut on and off for the last couple of years, mainly with Paper by 53 and Procreate, I feel a small update is in order. 

I've used the stylus with the iPad 3, iPad 4, iPad mini and iPad Air. I've noticed a gradual loss of effectiveness of the stylus - it feels harder to use resistance-wise than it used to. Could be a change in screen surface technology or just the wear-and-tear you'd expect from using a stylus. So my opinion of this has lessened to a degree. Perhaps Paper's 'Pencil' product will be the one for me, when it's finally available for non-silly money in the UK.