Why is my boyfriend so tired all the time?

My boyfriend and I have been dating for about 18months now and ever since I’ve known him he’s very tired.
We are both late 20’s and he works a stressful demanding 9-5 job and I live a fair way from him for my job so we only see each other on weekends but he’s ALWAYS tired.
Not just sometimes- all the time!
Even by 9clock he will be yawning and if we try to have s*x or anything like that he will only go once and then be too tired.
I really don’t get it, I’ve asked him to see his doctor and get blood work done but he won’t... why is he always so tired?
its effecting our relationship, I feel like we don’t get quality time because he’s always too tired!
Why is my boyfriend so tired all the time?
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