He is abusing it. Something that will have to be addressed in rehab. Trying to talk about it now with him may be futile. Consider it harm reduction for now if he is not drinking while at your house.
This may be bad advice but i was in his shoes. Only i had my court date re booked till after i got out of rehab. Legal problems can interfere or distract from treatment. They(rehab) want to limit all outside distractions. I wasn't allowed to use the phone for 30 days as an example. Getting the most out of rehab takes a lot of focus and hard work.
I met a guy in rehab, his drug of choice was cold medicine(for real). He never had a runny nose ha ha.
I have relapsed on pain killers after having a wisdom tooth pulled. Anything mood altering will be dangerous.
Speaking of DUI this is my second criminal driving offense which has resulted in a three year suspension, sigh.
Good luck