THis won't block all but a lot, Direct instructions to eliminate porn from results but then he may notice & change it back. Obviously more words you could add to the list yourself. There's also a setting in LW options to disallow mature content, but not sure how effective it is.

If it's pictures he is downloading then you could add extensions to the word filter such as .jpg, .gif, .png, etc. For videos ... well ... .mpg, .avi, .mov, etc.