Breathing techniques are really effective for teens - like the 4-4-4 breathing (4 seconds inhaling, 4 seconds holding, 4 seconds exhaling), it's simple and it calms the nervous system in just a few minutes. Beyond that, it's good to normalize stress instead of trying to make it disappear completely; you could explain to him that some anxiety before a test is normal and even useful for staying focused, but when it gets too much (hence the stomach aches and insomnia), he needs tools to bring himself back down. You could also look together at how he's preparing for his revisions - often anxiety goes up when someone puts off studying until the last minute, so a regular routine and short sessions rather than one big cram session the night before, that makes a real difference. And if it really persists, talking to a school psychologist or a therapist who specializes in teen anxiety would be a good idea, because sometimes it's more than just exam stress.