What to do

My son normally does his own laundry and he's 16, for some reason he didn't do it the other day so I decided to do it for him. Long story short there was some old and questionable underwear in it, size 8 briefs with characters on it and some other size 10 with random prints on it. I decided to throw it out and ended up seeing then back in his room yesterday. I know I am nosey so I went though his drawer and there were more of them, size 6 to 12 and looked well worn. When I confronted him about it he said they still fit. I have a feeling if I throw them out they'll just reappear. Any suggestions?