14 year old, muscle problems

Hi, I am just writing to see if anybody can help my son. He is 14 years old, and plays for Watford Academy, he trains 3 times a week and plays messages on Sundays. Almost every single time he is always in pain with his muscles, he says they are tight and finds if hard to move. He stretches well, he has had problems in his calfs, quads and hamstring. He is a goalkeeper and mainly gets these injuries when kicking a dead ball. Please could somebody write back , telling us what he should try to do , or not do. Thanks
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dlovell (watford through and through)