The easiest way to start out with Power Functions is to get an official LEGO set that comes with Power Functions. This will allow your child to really understand how Power Functions works.
If he is more into traditional LEGO building with standard LEGO System pieces, one of the LEGO City Trains with Power Functions would be the best options. They include all the Power Functions elements needed to operate the train, and he can also reuse these parts in his own custom LEGO creations once he understands how they work.
If he is into advanced mechanical builds, the LEGO Technic line is a great option. Some LEGO Technic sets come with Power Functions already included, but they tend to be large and expensive. Others do not come with Power Functions, but alternate instructions for adding Power Functions are made available either in the instruction booklet, or at LEGO's website.
Some LEGO Creator Expert sets also have alternate instructions with Power Functions added. The #10257 LEGO Creator Carousel and the #10247 LEGO Creator Ferris Wheel are some examples. Again, with these sets you have to purchase the Power Functions elements separately, but it is made very clear what is needed and how to install the parts. (LEGO Power Functions components are available individually at LEGO's Online Shop.)
Please note that currently there are two LEGO Power Functions systems on the market, and they are NOT compatible. One is the original LEGO Power Functions system (now referred to as LEGO Power Functions 1.0), and the other is LEGO Powered Up (also referred to as LEGO Power Functions 2.0). The new system has similar components, but they are not interchangeable, and they cannot be mixed. The new system works with Bluetooth wile the old system uses IR. The new system can be operated either by a traditional remote or an app. The old system only works with a remote.
So, it is a good idea to make a decision right now which system you want to invest in, and pay careful attention that sets and components you buy are compatible with the system you choose. In general, you can power LEGO sets with either systems, even if it calls for the other system, although some features may not work, or not work exactly the same. The components have a similar size and shape (i.e. you can install the new Power Function system in sets that were designed with old Power Functions in mind).
As far as resources for custom Power Functions projects, I found YouTube to be the most helpful. Power Functions can be a bit complex, so seeing exactly how something is installed can be very useful. Once your son learns the concept of how Power Functions works with an official set, he will start to have his own ideas of what he wants to power. At that point, he can do a search on YouTube, and will probably find someone who already worked on a similar project.
One of the most well-known and respected LEGO Technic and LEGO Power Functions builders is Sariel. His YouTube channel is full of tips, tricks, custom builds, and reviews of official LEGO Technic and LEGO Power Functions sets. His channel is professional and child-friendly: https://www.youtube.com/user/searme/videos
To give you an idea, here is a video by Sariel with some more advanced tips and tricks for using LEGO Power Functions: https://youtu.be/vfIBGIIleCI
