What should I do about my 16 yr. Old brother, relieving stress and kind of getting it everywhere?

My 16 yr. Brother, as any 16 yr old, likes to relieve stress (if you catch my drift). My twin and I have been finding stale, white outlined stains on pillow cases, bed sheets, and the couch and the couch pillows. Pretty much everywhere. I don't just want to simply walk up to him and tell him about it cause I know he would be embarrassed. I just find it very gross that does that where everyone sits and sleeps. I would like serious answers and know how to get him to know I know and how to do it somewhere else and do it with like a designated towel or something and thoroughly clean up and sanitize afterwards.