Not advising you to use paint of any kind on that kind of shiny hard plastic that a kid could eventually put in his mouth. Sorry, though as your idea to paint a hero is very clever.
If he were old enough or an unusually disciplined child you might use a magic marker or model paint or hammerite.
Suggestion: Maybe you could instead etch or scratch a face into a duplo block if you are good at artsy stuff. There's no danger of playing with something plastic that's been etched into.
Hope this suggestion helps.
