Basically with Matilda swimming was learning to put their heads under and learning to be comfortable with floating and using their arms & legs in the water. And the age group she was in was up to 15 months and the next level up the only difference was using "arms" so instead of them just falling into the water they were lifting their arms up and diving in iykwim? They also were just a bit better at everything but were doing really similar stuff. So he should be fine

And I wouldn't worry too much about what you wear, I wore a t-shirt and shorts as did another mum, one dad wore speedos and one other mum always wore a bikini...LOL so it was up to us what to wear... no one said anything different to us O