We own a room upstairs of our house and we have provided it for rent for a 23 year old boy who is working. This boy does not have any bad habits and addiction. He is very well mannered. Once I entered his room in his absence for maintenance purpose. I was shocked to see panties in his room. Since then I started observing him. He always wears panties and he is always nude on the bed. Is this normal?. Shall I let him know about his wrong things or should I keep quite? Please advice

This is sexually deviant behaviour. It is known as Transvestism. Transvestism is the practice of cross-dressing (wearing the clothing of the opposite sex). More common in males. They simply enjoy wearing female clothing at times, and most admire, and imitate, women.
You should take him into the confidence so that he would not feel offended and suggest to take the help of psychiatrist. This habit will not go by simple advice.