I do emails as well with my therapist and more than once she's missed one. I use LivePerson and their email format causes complications. So, it's perhaps possible there's a technical reason or some type of emergency/trip/unexpected commitment he's been dealing with as well.

Regarding your message, I think you should send it. It's good to be honest, and his job is to encourage that and help make things easier for you emotionally, so... hopefully he'll appreciate it!

I hope it goes well, bravery usually pays off for me with my therapist.