Many alcoholics do relapse. My husband was in rehab for 31 days, had 38 days sober, and he relapsed these last 5. Rehab doesn't guarantee sobriety. And sobriety isn't the only goal here - recovery is the bigger goal. He has to want it and he has to work it. I'm realizing that now. I'm also realizing that relapse is quite often part of the process. Some people come back from it and some don't. We can only hope that our loved ones are the success stories and try our best to support them through their recovery.
Worrying about him won't change anything. Trust me, I come from a long line of adept worriers - nothing good comes from it. Educate yourself. Arm yourself with information and tools to help you navigate this tough time. A very eye-opening thing for me was watching Pleasure Unwoven - it's available in full on YouTube, and it's a great insight into addiction and what it does to the addict, physically and mentally. It's a debilitating, terrible disease, and it can't be cured, but recovery is possible. And it's possible for you to work on your own recovery as well.
This forum has been a godsend for me. There are so many here who can understand how you're feeling and what you're going through. I wish I could make you feel better, but all I can do is let you know that you're not alone and encourage you to keep reaching out for help and support.