Ok, I did it. I emailed him. I wrote the following (translation's a bit weird):

"Hi,

This is Neutrino.

Hope you've had a good vacation. I remember you told me you'd be back at the office today and since we agreed on getting in touch with each other to book a new appointment I thought I'd email you to see when you've got time. What day would suit you?

Thank you,

Neutrino"

I hope that was good enough. I guess the ball is in his court now. Thanks for helping me out with this. I know contacting your therapist is a seemingly simple thing to do but I think it's very difficult. Anyway, it's done now. That's good.

Oh and thank you, Mastodon, for wishing me good luck. I'll need it. Exams make me anxious.