we started swimming when DD was 4 months old, there's a parent and baby class at our local pool two mornings a week, they have floats, balls and toys etc in the water, and they do lots of nursery rhymes where you're pulling the baby through the water - Jen loves it! we're not encouraged to use armbands, but they have those noodle type things we can use - DD is now 22 months and almost swimming alone (if only mum was brave enough to let her go!). if we go at other times, we take a ball and a noodle with us, but sometimes they'll get stuff out for you if you ask the lifeguard.
make sure you have a warm towel on the side of the pool so you can grab it as soon as you get out. if baby is shivering when you get to the changing rooms, maybe hold him under the shower, if it's too hot, you can stand under it and splash it onto him - warms them up a treat, and just get him dry and dressed as fast as you can.
we use huggies swim nappies, mainly because she grew out of her reusable and i haven't been able to get another one, but i think Mothercare do the reusables one for about five pounds, you just swill them out and then throw them in the washing machine.
also - when Jen was tiny, if she was tired, she sometimes fell asleep in the pool, it doesn't matter, they're safe with you! I always have a drink and a biscuit ready for when we get out, keeps her busy while I'm getting dressed!
hth - hope he loves it, it does wonders for their confidence!!