Get him somewhere besides msn, never got so much spam and porn spam as I did with a hotmail account. My mother had msn and it drove her crazy, she's got a local service now working much better.
Do need to check for viruses tho, could easily be sending from his computer without his knowing it. But it is strange that msn's email may not be virus protected in any way. I know my yahoo account won't let viruses in. when I get an attachment that is a virus with it, the attachment won't be there just looks like it is. They don't let that stuff in to begin with.
Yeah get him off msn, there's better services almost everywhere. Find out what life is like without 500 spams a day.