what sort of art is this ???

[seatrout]

This is a pic that my 12 year old grandson drew for his grandad .. good eh?

I want to make a scrap page but I wondered if anyone knew what this sort of art is called - I know a lot of kids are into it at the moment but don't want to show my ignorance too much by asking him

Subway art / graffitti art ????????????

Any suggestions would be welcome

Thanks