It's difficult to explain in writing, Myermay! All the lessons kind of build on each other and you don't notice what they're doing IYSWIM.
What do you do with your son? Do you encourage him to jump in? get him to kick and paddle and hold onto the side? A lot of early Aquatots is about water safety. The baby is basically "trained" that, should they fall in, they turn back round and hold onto whatever they've fallen from. Also, they learn to push up from the bottom of the pool to get their head back above water for a gasp of air which can give them a vital few more seconds for someone to get to them.
At 18 months I think that in Aquatots you do things like walk backwards across the pool holding your child at arms length. You then say ready go, dunk them under then bring them up, watch them take a breath and then do it again. This mimics the action of swimming across the pool underwater but coming up to breath.
You teach them to paddle with closed fingers, encourage them to kick (involves hoding them and saying "Kick kick kick!" Makes you look aright weirdo in a public pool!)
When they're making a good effort at kicking and paddling, you do the walking backward thing above, but when they'reunder water, you let go! Keep walking back and hey get pulled along, you lift them up, and say "breathe" and do it again when they've taken a breath. Then you introduce a tap on the back of the head when you say "breath" before lifting them up to breathe. The idea is to teach them just to lift their head back to breathe. They will not have the physical ability to do this until between 2 and 3 because of the strength and coordination required to paddle, kisck, tip your head back and breathe allat the same time - the idea is to ingrain the moves into them and then one day, suddenly, they put it all together and swim (at around 3yrs).
It is far far better to have this all shown to you. Try to find a class near you - you could probably find one that will take older babies and your son may well skip through the lower levels because of his current confidence and age-related ability.