I’ve been pretty happy with Personal Brain and, in fact, bought it because its cross platform. If he uses the “take a picture of the web page” style of web clipping from OneNote, he’ll want to use the Mac “Grab” application in conjunction with any of these. These are generally more full-featured than OneNote. Is there a native mac application that would be good replacement for OneNote - especially its notebook metaphor and web clipping?
Up till now, he has been using OneNote fairly successfuly to organise himself and grab web images and text for research and the like. My son has recently commenced a tertiary course requiring a macbook for film & image editing.