You seem to be getting it...understanding exposure seems to one of those things that is a mystery at first but then once you "get it"....Eureka! Getting it right, is still a bit harder but at least you can understand how it works.
It sounds like you are using full manual mode, is that right? It almost seems like you are picking your settings randomly...until you come to something that works. This will definitely help you to learn...but it seems unnecessary. You don't have to know the specific settings to use in a situation (although that may come after plenty of experience)...what you have to learn is how to meter a scene.
Your camera has a built-in meter, learning how to use it properly is the key. I like the idea of using one of the priority modes, either shutter priority or aperture priority. This way (for example), you set the aperture and the camera's meter will give you a shutter speed for what it thinks is a good exposure. Same idea with shutter priority, just reversed.
Now, the trick is to know what your camera's meter is seeing...and knowing that it will always try to turn everything to mid tone or middle gray. So if you metered something black, the meter would give more light to make it grey....if you metered something white, the meter would subtract light to make it grey. We obviously don't want everything to be grey so you adjust (or compensate) the settings to make the exposure look closer to how we actually see it (or how we want it too look).
The default setting for the meter is probably a matrix, which would take most of the scene into account, maybe giving the middle more weight (centre weighted metering). To get an accurate meter reading, you should zoom in or get close to something that is uniformly toned...like grass or someones shirt or face...or you own hand, if need be. Then you take that meter reading and compensate the setting...based upon the what you metered.
Grass or green foliage, for example, is about mid toned...so meter that and then don't change your settings. You son's skin is fairly light, so you could meter his face and then give about one stop more light (slower shutter or bigger aperture). You give more light because the meter wants to subtract light to make it grey.
Learning how & what to meter can be tough. I took a class, specifically about metering, and I still find myself trying to figure it out, when I'm shooting.
Sorry for rambling on so much...I just had a very large cup of coffee.
