For things like the designs on diapers, I think you could just explain why there's different designs and that they all serve the same purpose! They are a lot smarter and intuitive than we think, so you could try explaining that they are all diapers and serve the same purpose - you may not like the design on it but sometimes, in life, we aren't given the privilege/option to choose. Let him choose something else like the design of the pants that he wears with the fox diapers, making it a "unique/special" one instead of the regular elephant :) Alternatively, just get diapers without special designs!
As for clothes, I don't think there's an issue because nobody ever says that clothes/socks have to match! If he's happy wearing striped on one leg and red on another - so be it :) I see it as a way of being creative than sticking to the norm, small things like that which may make the difference between raising a child that sticks to what society does, and one that does what he loves :)