What Causes Intermittent Pain In Testicles?

My son is 18 and has been having pain in his testicals off and on since he was about 15. I have taken him to the doc the ruled out hernia and a twist. He now has pains but it doesn t happen all the time and when it does he can hardly walk. When it happens it usually happens to one and not the other. He told me today that he has noticed that he gets it when he wears boxers for a while. Does he just need to wear tight underwear from now on and not boxers? Is there a certain type of underwear that is better?