In the end I found a digger panel on eBay (i'm not sure what words I was using to search the first time, there were loads when I looked again!) It had some blue around the edges with various "digger" words. I then found some fabric that was clearly from the same range as it was blue, with words and little pictures of diggers all over it! Couldn't have been more perfect! Although I am now considering using the fabric as the backing as then he could have it whichever way up he fancied at the time!
I've never used panels before but I can see how they make a quick and simple quilt, that will look fabulous! Maybe I will make more child quilts!
Also, i'm not too worried about them growing out of the characters/theme. I'm 26 and if someone had taken the time to make me a little mermaid quilt, I'd still love it!
As always, thanks for everyone's comments and suggestions :-)