Ask for pin when switching profiles on XBox-one?

There is no way to prompt a pin when switching profiles. You only need to input your pin when you sign in and since you already signed in so they could use your account, you wouldn't need to put your pin in again.

But there is a different way to give him access to your gamepass privileges. If you set the Xbox as your home Xbox then your Xbox Gold status and all of your games will be shared with everyone on the Xbox. To set your home Xbox Select Profile & system > Settings > General > Personalization, and then select My home Xbox. This should allow your son to play your games even when you aren't signed in.