The intron a drug should not be used with rebetol drug.The serum levels of thyroid
hormone, blood counts including hemoglobin, complete and differential white blood cell counts, and platelet count should be monitored.
The serum electrolytes, liver function tests, and TSH should also be done periodically to know any developing abnormality.The chest x-ray may also be repeated if needed.
He should get his haemoglobin levels checked as the anaemia may also cause the fatigue
The drug intron A does cause "flu-like" symptoms, as the side effect particularly fever,
headache, chills,myalgia and fatigue.
It is likley that he is getting these symptoms of fatigue as the side effect.If the symptoms worsen or are not controlled then consult the doctor.
The risk to benefit ratio has to be kept in mind while one is on the drug for long time
Here is the link for more informationside effect of intron A
Let me know if you have any doubt.