Cancel Subscription for Simply Piano on family sharing?

Hello and welcome to Apple Support Communities, Martin!

We understand you're trying to cancel your son's subscription. We'd be happy to provide information about this.

Based on the screenshot you've provided, the subscription has already been cancelled. Notice how none of the options are checked in? This would indicate that it has already been cancelled. This article mentions that if you don't see the cancel option, it has already been cancelled:

How to cancel a subscription from Apple

"How to cancel a subscription on your iPhone, iPad, or iPod touch

1. Open the Settings app. 
2. Tap your name. 
3. Tap Subscriptions.
4. Tap the subscription that you want to manage. Don't see the subscription that you're looking for?
5. Tap Cancel Subscription. (Or if you want to cancel Apple One but keep some subscriptions, tap Choose Individual Services.) If you don’t see Cancel, the subscription is already canceled and won't renew."

Have a great day!