As a few have already said, you might be better not using your flash, but if it is allowed it might be worth having a play with it during the warm up - if you're not happy with the results, or not confident using it, then don't. I took some at a couple of recent galas for our local swimming club, my first real attempts at swimming photography indoors, here are some of my pictures
https://photography-on-the.net …/showthread.php?t=1351840
As I said, these were my first proper attempts, so I did some research on the net to give me some ideas for settings, and I finished up using my 60D body with a 70-200 f4 lens, RAW, AV mode at f4 with the ISO at somewhere between 1600 to 6400 to give me a shutter speed fast enough to freeze the action. Focus mode was A1 Servo, focus point was either centre or extreme left / right of the diamond shape, depending on how I wanted to frame the shot.
Another tip is to take plenty of shots during the warm-up to check your settings, and finally...make sure you have a programme of events, this way you can make sure that you're in the right place at the right time for the stroke your son is swimming and also for the lane he is swimming in.