Five year old diarrhoea when swimming

[shouldbeworkingtoday]

i haven't posted before, so apologies if any of this isn't clear, or in the right place...

My five year old gets the terrible squits almost every time he goes swimming. I think his stomach is reacting to the tons of water he swallows but it's getting to be a bit of a problem. I put a happy nappy thing on so it contains it if it does happen but it makes an awful mess to get him changed afterwards.

He doesn't have any other bowel problems, no trouble potty training or anything and no other issues with the toilet or his stomach.

Wondered if anyone else's dcs have/had the same problem, what you did about it if anything. Did they grow out of it, is there anything I can do to prevent it and if anyone has any ideas about how to limit the damage? I don't want to use a nappy since it's going to be humiliating for him but it's obviously distressing for him as it is!

Help!