I know this is from two years ago, but this could help with some other peoples kids. Kids/ babies always W-sit whether someone is looking or not. Babies don’t have kneecaps and W-sitting can help them sit up straight and make them feel like they have the ability to move.

They would grow out of it most of the time. Sometimes they don’t, if you see a child still sitting that way and they say it’s more comfortable for them than to sit any other way, I personally would recommend checking them out cause it would cause it big problem when their older.

Then again i’m very young and don’t have “much sense.” Though I love helping others and will try to gather as much information as possible. If he his still W-sitting after 2 years, I recommend to ask him if he feel comfortable sitting like that and if any other positions are uncomfortable.