It depends. Many employers will work with you if you seek treatment proactively, before they have to mandate treatment. If he has short-term disability he can probably file for that. The way mine works is I had to exhaust my vacation and sick leave fist, then the disability kicked in. If he works for a large company he probably is eligible for FMLA which will protect his job for 12 weeks. It's a rare employer that would seek to terminate during this period due to the liklihood of litigation (it is taboo to fire someone who is on medical leave, which is what rehab is).
Best of luck. I went into rehab for severe depression back in February. I was hospitalized for 7 weeks. There were many MANY people in for substance abuse. My hospital will advocate with your job as long as you are maintaining treatment and you don't relapse. I am in a therapy group with all nurses. Some of them were mandated into treatment for being high or drunk on the job or for diverting drugs(into their pockets.) Some of them have been able to go back to their place of employment, although they are not allowed to give, handle or be around drugs. Some of them, mostly those who encountered an intervention at work, lost their jobs and have been able to find new jobs. One of the commandments is that they must reveal their status as a recovering addict to any potential employer. It's surprising how many employers will work with you. They really like it when they find out that you sought treatment on your own, rather than being forced into it.
Please be patient with your loved one. Rehab is incredibly hard and there is a high likelihood of relapse. Most people have to relapse 5-6 times before they finally get clean. That can be very frustrating for the families. If you're not an addict yourself it's very hard to comprehend how hard it is to quit. Addiction is a disease not a character flaw. Best of luck to all of you.