The best thing you can do is to support him by being patient, understanding, and tolerant. His moods will be all over he place for a while. His anxiety will be higher than usual so accept that he may be restless, inpatient, and snappy. No doubt he will go through depressive phases, just listen when he needs to vent. He's going to have cravings and when they set in try to keep him occupied.
When he gets out of rehab he's going to have a lot to work through. Hopefully, he knows by now what is driving him to use. He is going to have to come to terms with his addiction, and what he's done while addicted. There will be a lot of remorse and guilt. He may or may not discuss this with you. Try not to say things about the past, or throw behaviour in his face in a negative way should the opportunity arise. Hopefully, he will have new boundaries - try to respect them. When I got healthy I had to re-establish my relationship with my family because the way it was before wasn't working for me - it was a huge trigger. He may want some things to change, and as long as the requests are reasonable, try to honor his wishes.
You may find that he my seem distant, don't take it personally as he has a lot to work through, and it may be upsetting and painful for him. He may not want to discuss it, so don't continuously pry if he doesn't want to talk about something.
Hopefully he will be in aftercare, either meetings, therapy, outpatient rehab - try to be ccepting of these appointments as they may not always be convenient. Getting out of rehab is really just the beginning of the recovery process. He is going to have to relearn how to live life, and it's not easy.
Some other things to keep in mind are that he should never be hungry, angry, lonely, or tired as these are all feeling that can trigger him to relapse. It's best that he be on a sleep schedule, go to sleep early, wake up early, and also be on an eating schedule. Eating healthy - like clean eating, really helps with the recovery process.
The only other thing I can think of is to try to break old routines and establish new ones. You may want to consider rearranging the furniture in the house, and changing up as much as you can so he can build new associations with a healthy lifestyle. Feel free to hit me up if you have questions. Good luck to you both!