I like the idea of letting him customize it. I think this may bump the Diamondback down the line again. Once I've got my wife's Trek done I think I'll bring it down and start stripping it for real.
I knew I'd seen a thread on here where someone had done something similar, but it really took me a lot of searching to find it, since I couldn't remember what the bike was:
I hung the frame outside on the washing line and told them I wanted them to make it undesirable, as it was to be my pub bike cum shopper, frame is ex-skip.
The base was a poundland rattle can, the rest was whatever was in the shed.
It ended up black with red spots all over, ladybird style. Have had many comments on it and a nice story to share.
...and don't forget the masking tape for nice tiger stripes etc. Of course, if you lived where I was brought up by 8 he would already be proficient with a spray can...