As noted above, absolutely avoid using the flash in such a situation. It is distracting to other viewers, and to the performers, and can actually be a safety hazard. Most venues will stipulate no flash in their entrance policies. So, use a mode which will not automatically trigger flash. I would suggest the "Tv" mode, which is "time-priority" or "shutter-priority". In a situation like this where the dancer is likely to be lit far differently than the rest of the scene, using a more selective metering mode like Spot (small dot around the focus point is used to determine how much light to let in) or Partial (slightly larger dot around the focus point). These are not the defaults (that would be "Evaluative") so you will want to change that setting right away.
Take the below with a grain of salt according to your comfort level. I am answering after having done this kind of thing for sixteen or so years with older daughters and younger daughters all of whom have had events or sports in poorly-lit situations like a dance auditorium.
I would do my best to get some test shots well before "the moment" arrives. The point of the test shots will be:
- Determine where you can stand in the theater to get a good angle on the performers (especially one which will capture their faces) without others being in the way.
- What shutter speed is necessary to "freeze" the dancers in motion. If this was a more frequent occurrence, I'd encourage you to play with slower shutter speeds to capture more of the fluidity and movement, but the most important thing in a one-shot deal is going to be being able to see your son's face clearly.
- How uniform will the lighting be in specific areas of the stage, so you know when you are most likely to get the best keepers.
- What range of zoom will you need to capture the full event, ideally without switching lenses. If you had multiple lenses with overlapping zooms, you would choose the one which has an acceptable wide angle and good fast speed (ie, a low f number for the aperture), even if that means you will need to crop the photos on your computer after you are done. In your case it sounds like the 18-135 is the only one available (unless you have a friend with Canon lenses you can borrow), but knowing when a wider angle will be possible will help you when in the heat of the moment trying to capture everything.
- How far "overexposed" can you go without blowing out the highlights that matter, in the dancers themselves.
Obviously if you can't take test shots then you will have to guess or trial/error all of the above (if there are multiple dancers, you can do your test shots on one of the dancers who goes early, then fall back to the back of the room to review on your camera screen what you got).
Once you know that ideal "freeze time" shutter speed, I would switch to Tv mode, turn the top dial to attain that shutter speed, and leave the rest in auto (including auto ISO). Essentially, this will end up keeping your shutter as wide open as possible with as low of an ISO as possible while still keeping the shutter speed fast enough to freeze movement.
If you are comfortable using "back button focus", I would highly encourage that. However, do not take this opportunity to learn it (it takes some time getting used to and people will tend to miss shots while learning it).
Change the Drive Mode to High-Speed Continuous. This should be around 7 frames per second clicking off for as long as you hold the shutter button down, which will allow you to capture a full move without trying to press a button over and over again multiple times per second.
I would leave the autofocus on "AI Servo", especially if using back-button focus. Set the focus mode to point, and specify the center (most sensitive) focus point. In dark settings like this it is critical to have a good sensitive cross-type focus point, which the center point always is. Without BBF, AI Servo means that while holding the shutter button down the camera will keep trying to get focus on the center. In either case, your job once you start holding that shutter down is to keep your son directly in the center of the frame with the focus point over him.
If you are comfortable doing post-processing with a tool like Lightroom, set the image type to "RAW", not JPEG. This will require you to do some post-processing after the event, but will allow you to do that processing with all the data the camera captured rather than the small subset committed to a JPEG. RAW will limit how long you can hold the shutter down before the capture rate slows down (and that is also a function of how fast your SD card is), so keep that in mind (I believe the 70D is rated to capture infinite number of JPEGs in high-speed continuous mode, but something more like 2-3 seconds' worth in RAW capture mode). Also having a large SD drive is imperative when capturing in RAW as the images are easily 10x as large.
While shooting in RAW, the "white balance" is not important, which is also a great reason to use RAW. Your camera will record what it thinks the white balance should be so Lightroom can default to that, but unlike JPEG you can easily change it (especially if there are different color gels being used across the stage) or make all photos use the same temp/tint settings. However, if using JPEG, I would try to set the white balance beforehand according to the type of lights in the theater so things look consistent from shot to shot.
I would also leave the "IS" on the lens on. There are no known drawbacks to having IS on in Canon lenses (there are on Nikons), and the extra bit of stabilization will help even if you are shooting at a 1/500 or faster shutter speed.
Beyond all that, don't forget to have fun and enjoy the show. There are too few of these opportunities to come away with only pictures and no memories. An imperfect picture will trigger a great memory and be a conversation starter, but if you didn't really watch any of the show in the first place you have no memories and nothing much to talk about afterwards.