He's moving out in a week, it looks like. If you're comfortable keeping the keys close until he moves out, I guess it makes sense. Although if he is using the car to go get drugs, there will probably be some type of explosion within a day or so- so you need to be prepared to ask him to leave sooner than September 1.
The boundary of not wanting to allow an addict access to your house is certainly reasonable...so changing the garage combo and asking for his key back makes sense to me. If you think he may have made a copy of the key, though, you may want to change the locks as well.
If he is in recovery, he'll understand. I remember when my son asked if it was "okay" for him to have a key again - he was definitely prepared for us to say "not yet."
Best of luck.