I would say fear of the unknown is normal. He is almost certainly feeling it too. Perhaps you don't have to put it out of your head. I find mindfulness meditation an absolute gift for when my SO is newly out of rehab. Also, bear in mind his life is not saved yet. There is a very very very long journey ahead for both of you and relapses are almost certainly part of it.
I would say also bear in mind that it will be al several months before he gets his head back together so don't push for deep talks unless he initiates or it is absolutely vital. You will both need space to process and to be with yourselves. I found it helpful to tell him that and to take the pressure off by saying I didn't know how to handle it either. Long walks together up hills helped us spend time together without pressure.
Baby steps, open non-judgemental communication and plenty of space, not interfering in or asking questions about each other's recovery but rather each volunteering what we were ready to share.
Most importantly, let yourself feel what you are feeling. It's helpful to get to know your own needs. Seriously, mindfulness meditation has helped me all three times he's done rehab.