When I'm swimming I bring a (plastic) lucozade bottle in and put it at one end of the pool. You can then take sips at the end of each double length if you need to, just to keep the blood sugar topped up.
I do understand why you don't want to mess with the dose but eating the extra carbs is also messing with the blood sugars really because you can easily end up too high. I would recommend giving it a go to reduce the insulin, depending on when your son is taking his jabs. Eg if he's eating then going to swim shortly after, just reduce the dose by 1 unit and see what happens.
Everyone's different, but just to give you an idea, I would normally take 6 units in the morning for my breakfast slice of toast and jam. On swim days (32 lengths) I take 4 units to get the same effect blood-sugar wise.