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Yes, he should be seen. The persistent diarrhea may be the reason that he finds it difficult to maintain an appropriate weight and it would also contribute to his fatigue.
If his diarrhea has been ongoing for at least 4 weeks then it would be called
chronic diarrhea.
There are different possible causes of chronic diarrhea such as: (1)irritable bowel syndrome
(2)
inflammatory bowel disease such as crohns or ulcerative colitis
(3)infection
(4) hormonal causes such as
hyperthyroidism (high levels of thyroid hormone). This would also affect weight
(5)metabolic conditions such as
diabetes, which can affect the nerves of the GI tract and therefore cause diarrhea
(6)side effect of medication
(7)
food allergy or sensitivity to particular foods
To find out the exact cause , he would need to be assessed by his doctor. This would include physical examination, stool sample to the lab and blood tests.
Depending on the results of the above, imaging of the intestines may also be needed.
I hope this helps