There may be three or four reasons why you get blurry shots. Posting an example of one of these shots will help us identify your specific situation (especially if you can post the image so that all of the shooting information (aka "EXIF data") is in the image that you shoot. Specifically we would want to know what focal length, what aperture value (f-stop), what shutter speed, and what ISO setting was used when the shot was taken. But there are also other bits of information in the EXIF data such as which shooting mode was selected, which focus mode was used, and which focus point was used to establish focus.
A picture would help... but a picture that included all of that shooting info would help a lot more.
1. You may simply be missing focus... the camera used a focus point which caused it to focus on a target OTHER than your son.
2. You may have been using a very slow shutter speed and your body was not perfectly still when you took the shot. This is blur caused by camera motion.
3. Even if the camera did not move, you may have captured a shot when your subject WAS moving ("subject" motion instead of "camera" motion).
Sometimes blur is actually desireable when shooting "action" because it's a way to imply a sense of motion or speed in a still photo (it is used as a creative effect).
But I will guess you are probably shooting indoors and that means there is not likely enough light to allow you to use a high shutter speed. Slow shutter speeds often result in blur caused by camera movement when the shutter is open.
Tripods and mono-pods can help with camera motion, but there are also techniques to allow hand-held shots by learning to stand and brace the camera in such a way that body motion while shooting is less likely to be a problem. Also if there is anything you can lean on (a wall, a support column, a door-frame, etc.) this can also help you reduce any camera motion.
You can also select a lens that can collect more light when the shutter is open. These lenses have lower focal ratio values. For example, the Canon EF 50mm f/1.8 STM has a physically larger aperture opening than your EF-S 18-55mm f/3.5-5.6 lens. At the 50mm focal length it collects nearly 10x more light than the kit lens you have. That translates into allowing you to select a shutter speed which is 10x faster than you can use now.
Another possibility is to simply increase the ISO setting on your camera. This comes with a trade-off which is that images that are shot at very high ISO values will often appear to be "grainy" (that look is referred to as digital "noise" in the image.) But better to have an imaging that looks noisy/grainy (technically "grain" is a term referred to actual photographic film photography and "noise" is what we call it when it's a digital image - they are actually not the same thing, but they have a similar look so sometimes people use the terms interchangeably.) To avoid the "noisy" nature of high ISO photos, we usually prefer to shoot with the ISO setting lower whenever we can. But when given the choice between a "blurry" shot and a "noisy" shot, I'll take the noise (you can actually do things in post-processing adjustments to reduce the noise but there's little you can do to fix blurry shots.)
There is one other possibility.
If you are using the camera in the default "One Shot" focus mode, then the camera can achieve focus on a subject... but then the focus distance to that subject changed. The camera will not update the focus in this mode. You have to select "AI Servo" focus mode if you want the camera to use continuous focus to compensate for subjects where the focus distance is changing as you shoot. There's also "AI Focus" which tries to auto-decide which focus mode should be used for each shot.
Tim Campbell
5D III, 5D IV, 60Da
