I started at a new job 4 months ago, and I really think that my new workplace did a really good job on this new hire stuff.
They have a so-called "buddy-program", where developers can sign up to be a buddy / mentor for new employees. When a new developer is hired, a developer in the same team (who signed up to be a buddy) is pointed out to be the new guy's buddy / mentor. It is his responsibility to get the new guy up and running, help him out with the problems he will encounter, go to lunch with him, introduce him to the team and so on.
B. Low dependency
This was previously mentioned also, but start out with assignments that are not critical and too hard to do, without full knowledge of the code-base. There is nothing worse than getting thrown into a code-base you don't know, and then need to perform from day 1. If possible, make assignments that are relatively easy AND span across the entire system. That way he will get some knowledge of the system.
You probably have small tasks that you didn't have the time to do, when you first wrote the code. Give those tasks to him.
C. New-hire trainings
This is probably luxury to most, but I work in a large corporation that can afford to do this. Every month or every two months or so, some kind of new-hire training are arranged. It is basically a series of presentations, with the goal of giving the new-hires some kind of knowledge about the system, how testing is done, how the bug-tracking works and so on.
Where I work, there is a lot of different teams, working on completely different tasks, but on the same piece of software. A member of each team is assigned to do a presentation on their part of the software, where they give an introduction to what that part does, how it works and so on. Besides that there are trainings on tests (I am working as a tester) about how tests are written, how they are run, how they are checked into the codebase and so on.
To sum it up, be gentle on new-hires and don't expect serious work from them in the first month or so. Give them tools to get help and if possible, give them some presentations or videos.