My son gets dehydrated after play hard, he gets a really bad headache and vomits.could it be more than dehydration? he is an average healthy 9 year old. who will play very hard and forgets to re-hydrate himself. my resolution has been to give him liquids

I : I think it would be best tha the have a medical evaluation including blood and urine testing to be sure he doesn't have an underlying condition causing this, such as elevated blood sugar or other conditions. His symptoms are unusual so a medical evaluation would be prudent. Please write down the symptoms and tell your pediatrician.

Sunscreen/hat: He should stay hydrated, a dilute sports drink mixed with water may be provide to replenish salts. A waterproof sunscreen and a hat are recommended.

Pre-load: Any kid with such a problem should be encouraged to load fluids during the period before any such exercise/play. If you must pay him with extra device time or other bribe, do it. If this does not help after a trial, have his doc evaluate him. There is a rare but worrisome condition where the kidney fails to retrieve water normally leading to such symptoms (diabetes incipidus).

H/A/ vomit s/p exerc: We have a thirst mechanism that makes us want to drink. Are fluids available on field? . In cold weather or warm weather ? Dehydration vs heat exhaustion which can be more serious. On any medications as some may predispose. https://www.hopkinsmedicine.org/health/conditions-and-diseases/exerciserelated-heat-exhaustion. Any cardiac issues? F/u/ w/ MD and cardiology as some issues are occult