I have 4 sons ages 9, 11, 13, 15. I have told them since my oldest first asked that they are not allowed to wear boxers as underwear, I don't feel its healthy so none of my boys are going to be switching from briefs.

What concerns me most is why a mother of a 14 year old is telling him what underwear he may wear!
Giving your son some autonomy on issues of personal dress taste such as this is important at this age and parental interferences over what are such trivial issues can only water down a paren's authority over more important issues that could arise with raising a teenager.
Does it really matter whether he wears boxers under his trousers anyway?
Stick to the more important issues of parenting is my advice.

Nice to hear from a dad at last, especially in such an issue as this. What the hell does it really matter if the poor boy wears boxers or briefs? And no they are not any less hygaenic! I have four boys ages 19, 12, 9 & 1 (whose still in nappies of course!). The two elder boys wear boxers the 9 yr old has both. I wouldn't dream of asking what underwear they had on! I think its good your son felt that he could talk to you about such a personal thing, I just hope he still feels he can talk to you about personal things after you've laid down an iron rule about such a trivial thing that to be honest at his age is none of your business!