That clarifies things. As long as ex is abiding by the agreement, you can't FORCE him/her to do anything else. They have no obligation to communicate with you.
It is, of course, preferable that they do. Therefore, I'd focus on finding a way to make it easier rather than worrying about formal proof of delivery or anything like that - especially since you're asking for extra here (like taking son to skiing lessons even when he was scheduled to be with your ex). In fact, making formal actions - like certified letters is likely to make it HARDER to get what you want because your ex will see you as escalating things.
If he/she won't respond to non-threatening communications like email or phone, then you may have to just live with the fact that you're not going to be able to work out anything other than what's spelled out in the agreement. That's not great, but lots of people manage to live with it.