Why High Levels Of Fatigue? Is It Due To Strain?

My son is running cross country and at the end of last year started have high levels of fatigue very early in the race. He seems fine in practice and has actually been running faster in practice than he did in his first meet this year. He had multiple tests done at the end of last year, none very conclusive ( blood drawn, cardio monitoring, etc.. While he ran shorter distances (competitively) during the track season, he had no issues. His track training was much more strenuous and included 2 a day practices. We are not sure if it is diet related, physical or just from the anxiety of competition. The next meet we are asking him to prepare the same way he does for practice and hope this works (not nearly as formal prep). Any input would be appreciated.