Hi Greg and Rachel,
My daughter was diagnosed with Type 1 two years ago and she loves to swim! She was having swimming lessons when diagnosed at a small, local swim school where parents could sit pool-side. So I made sure the school manager and her teacher knew and took the testing kit and hypo tin with me. Her teacher was fab and kept an eye on her and would call me across if she thought DD Hannah looked pale or if Hannah said she felt weird. So that was great. She was actually back in the water two weeks after we got out of hospital.
However, that was a shallow beginners' pool and she only lasted another couple of terms until we needed to move on to a full-sized pool, which meant finding somewhere new. So we chose on basis of personal recommendation, and she now goes to a bigger swim school where I have to sit in the gallery to watch. But all the instructors know she has Type 1 diabetes and most are first aid trained, so if Hannah says she needs to get out, she does and I take the kit down to the pool-side. It has only happened once in 18 months.
We go on a Saturday morning, so she has breakfast and does insulin as usual. Then we drive to the pool and she tests just before she goes pool-side. Lesson is half an hour, and she has a snack and a test afterwards. We find that, for her, her BG level is usually fine after the lesson, but drops dramatically about half an hour later, so the snack stops that. But of course, everyone is different. We are starting with a pump this week, so will be "interesting" to see what happens with that.
We do some family swimming too, and I take the kit pool-side and hang it on the hooks near where the lifeguard stands - never had a problem when I've told them what its for. We have had a few hypos on family swims on holiday, so we just haul her out, get under a towel and test, correct and wait. We find fruit juice good for quick results. One lifeguard in France came over to see what we were doing and why she was eating, but as soon as he heard me say "Diabetique" he was fine about it.
So upshot of all that is - go and talk to the swim school. They should be able to handle it, and your diabetes nurse may offer them training too. At Carter's age (same as Hannah's when she was diagnosed), there should be an instructor in the water with them anyway, but look for a ratio of no more than 1 adult per 6 children. If you don't get the answers you want/need, then ask around anyone you know whose kids swim and see what they recommend or say. Hannah was keen to try the local swimming club, but a friend (who has a son who is a county standard swimmer there) warned us that they don't always have the same teacher and sometimes a parent stands in for a lengths session, and you aren't allowed pool-side at all, so we decided that wasn't for us.
When you are swimming as a family, take it easy, have fun and stay within arms length of him until he is swimming confidently on his own in lessons and you have a better idea of how swimming affects his BG levels. He can swim safely, he will have hypos in the water occasionally, but he and you will learn to handle them. Stay alert and keep calm - and try some buoyancy aids. DD didn't want to wear armbands, but games with a woggle/ noodle were fun for her and gave extra reassurance for us. And you can buy a thermal emergency blanket etc to keep in your hypo kit to ensure no-one gets cold if you have to get out in a hurry. Even at the beach on holidays, try to swim where there's a lifeguard - they can help if you need them.
Good luck, Carter, and happy swimming!