We just got a trampoline and the first day my 12 yr old son was fine. jumped for about an hour. then on the second day (the next day) his chest hurt.?

Splints!: Mild diaphragmatic irritation is common after a first bout of aerobic exercise. Both the liver and spleen can rub the diaphragm and have a temporary inflammatory response. This will dissipate with time. It should respond to nsaids. If the problem persists and does not improve, see your doctor!

Hopefully okay: Chest pain in children should be approached with caution, may be he over do it and minor bruise in the chest cage, if continue i advice you to check it out.