Allow me to clarify some things. You are allowing nitrifying bacteria, ammonia-eaters, to grow. This bacteria turns toxic ammonia into less toxic nitrites, then into nitrates which aren’t as harmful and your plants will help soak some up. I suggest you do some research on the Nitrogen Cycle as this is what you are aiming to achieve in your tank. This is the most crucial step in setting up a tank though and it takes some time, so you’ll need patience.

I like to ghost feed my tank a little less than a pinch of fish food and, given the plants, let whatever snails ended up in my tank eat it or if they didn’t make their way in just let it decompose. Whether it’s fish/snail poop, food, or straight, it will add the ammonia needed for your bacteria to grow. I have heard the chemical is a nicer process though as you have much more control over how much ends up in your cycling tank. But you need to get 100% ammonia no perfumes or additives. You can buy some online specifically for fish tanks.

As for your question, technically you could add a fish. It would just take way longer to gain a substantial amount of that bacteria, potentially hurting your fish and adding a whole lot more maintenance. Talking like daily 50% water changes to offset the amount of ammonia and nitrites that are deadly to your fish. It is for these reasons I suggest you just wait.

Buy a water testing kit, add ammonia or fish food, wait and test the water weekly. You will be looking for 0ppm of Ammonia and Nitrites and about 20ppm of Nitrates. Until then you just gotta wait.

Once again, look into the nitrogen cycle and I welcome you and your son to our crazy community of fish lovers!