It depends - did you tick the box on the DS-260 to ask for it to be issued? If so, it'll be done when he enters the US on his immigrant visa, and doesn't take long (ours were here within about 8 days after we arrived). If not then he'll need to go to a SSA office to apply in person.
Don't be overwhelmed, if you're there already then at least he's got you to help him do stuff! And it should be fairly simple if you already know how everything works in the US. I moved last year with spouse, two teenagers, two dogs, and we had to start from scratch not knowing anybody or how anything worked. It was all absolutely fine, he will be too.
Best of luck.