Is it normal for a 15 year old boy to hide dirty used pads and tampons in draws?

No, it is absolutely not normal for anyone of any age or gender to hide used pads and tampons - his parents should talk to him about why he does this and consider counseling. Some menstruating people may hide their used menstrual products when they first start menstruating, due to embarrassment they feel towards menstruation - but most menstruators are girls/women, not boys/men.