Agree that the noise may be a bit worrying. I take DD swimming every Tuesday morning and normally shes fine but this week it was busy and noisy because of half term and she became a bit clingy and upset.
Apart from that there's not much you need to know. The only tip I would have is don't take him when his tired or hungry and keep swimming to about 30 mins so he doesn't get too cold.
Oh great idea re the shower afterwards. We're going to a pool that (I have been reliably informed!) has family change rooms and DH is coming too, so I figure one of us can dress DS as the other gets dressed and then swap. Am looking forward to it!
Oh, we've got one of those Pampers swim nappies and also a set from M&S with the towelling bit - is it overkill to use both?
Oh Yes, some have hated it initially then been fine after a few times. Others have loved it initially, then hated it later on. A 3.5year old I care for has only recently decided he will go back to one of the pools near us... he likes it now but didn't like it about a year ago, so we changed pool.