It does make a difference, because if he left it in full view, then it can be considered a plea for a conversation to begin about his preferences. If you were snooping, then it's best that you inform yourself about the practice of bondage, but wait to bring it up with him until a later date. There are many ordinary married couples who have expanded the boundaries of their sexuality a little to include games that might be referred to as bondage and discipline. The gist of it is that these are fantasy sex games where one partner has complete power and control over the other, sometimes including physical bondage, for the duration of the game. For more information on consensual BDSM, see my answer to Gloria .
Tight constriction can create very intense stimulation.
You're all tied up, and all that's left is to enjoy. It's stimulating to struggle, to let your body lose control. It can really intensify an orgasm when every muscle is straining against your bonds as you're trying to get your hands free to stimulate yourself. What I would suggest is that you make an appointment with a therapist to learn more about your son's practices. Since I don't know exactly what you've found in his room, it's hard to explain what it does or why he may be using it.