He will be tired because he will be exercising so hard. This sentence emphasizes that he will be tired because he will be exercising at that exact moment in the future.
He will be tired because he will have been exercising so hard. This sentence emphasizes that he will be tired because he will have been exercising for a period of time. It is possible that he will still be exercising at that moment OR that he will just have finished.
If you do not include a duration such as "for five minutes," "for two weeks" or "since Friday," many English speakers choose to use the Future Continuous rather than the Future Perfect Continuous.
Be careful because this can change the meaning of the sentence. Future Continuous emphasizes interrupted actions, whereas Future Perfect Continuous emphasizes a duration of time before something in the future.
住戶編號:5840414
檢舉原因
心情: 
