Welcome to the forums and thank you for being here.
I'm sorry that you're struggling with so much anxiety; I've been there and it's a bit of a nightmare! I can see that you're trying to hard to tackle it even though it often gets the best of you.
You said in your post the logical way of thinking is that there's a very slim chance, and you're absolutely right. The problem is though our emotional brains like to rule the show, and every time you ring or check up on him, you're kind of feeding it (even if you don't mean to). The reality is, 99% of the time, he will be okay.
I'm wondering if you've ever thought about seeing a therapist? I see one and they've been really helpful for me. They can help challenge those thoughts and let go of that need to be constantly checking in. Even though going to therapy can be hard, it's so exhausting feeling so anxious all the time.