Why does this 5 year old get naked?

[kendrix]

Okay this is really awkward but my brother in law is 5years old and last week his mom caught him all alone in his room naked. His excuse was that he was gonna go to sleep. But he never has slept naked he shared a room with his sister. And today well as I type this I am spending the night and I'm sleeping in my brother in laws room with him,and his sister. I was sleeping well falling asleep and I heard noises so I peeked out my blankets to see what was going on and I catch him In his underwear and what seems to be a bra. Then I ignored it because it was really awkward then I look again and he's completely butt naked. I know he's just a kid and I don't mean to put his family down but he plays and watches alot of movies and games he shouldn't at his age. He also caught his father sleeping naked from what I hear.
My boyfriend his brother is worried it might me a psychological issue and I am very concerned about his education. And the fact that he knows more about video games than colors and letters. He is 5 years old. My cousin of 3 years knows how to could to 20 in Spanish and English my brother in law not so much. Again I am concerned about him. Is this a psychological issue or is it him just being a kid? Any kind of help would be useful. Thank you for reading all this! :)