Child With Pain In The Stomach. Had A Fall. Muscle Strain Or Internal Bruise?

My 6yr old son fell off his swing last week onto his front & said he bumped his tummy on the grass - he didn't land on anything - just flat on the ground. He seemed fine & recovered quickly & has been eating, drinking, urinating & defacating normally since. Over the weekend, he played on his new trampoline a lot. Then, a couple of days ago, he went to bounce on his trampoline & said his tummy really hurt when he tried to jump. This has happened several times since & he now says his tummy hurts when he presses it. However, he's continued to be well in himself & hasn't had any issues with eating, drinking, urinating, defacating, etc & his tummy doesn't hurt all the time - just when he exerts himself & presses on it. Could it be a muscle strain or an internal bruise? Should we be worried? Should we take him to the hospital? Any advice this evening would be very welcome. Thank you. Mrs Kathryn Green.