True that..Digital Rectal Examination.
Yet taking this patient's History and Age he might have a colonic stricture that may be due to :
left colonic carcinoma Low fibre diet since he worked for 20 years in a fast food restaurant and also the progressive course of unresolving constipation .
Another possible cause is Long standing Schistomiasis since he is from Egypt
Other Contraindicatio ns of DRE include :
Absolute:
Imperforate Anus,Unwilling patient,Immunos uppressed patient,Absence of anus following surgical excision,Modera te to severe anal pain,Prolapsed thrombosed internal hemorroids

Relative:
Severe rectal pain may preclude an awake anoscopy and require sedation,Recent acute myocardial infarction ,Acute abdomen,Patient s with valvular heart disease or prosthetic valves require prophylactic antibiotics before the procedure,Patie nts with coagulopathy (or anticoagulation medication) if biopsy is to be considered,Majo r rectal trauma,Post-operative status (anal surgery)

Why do you say so Dr.Katia ?

Well, coming back from Egypt, he might heard some bad news from his family in which causes stress or hypertension. The stress might triggered autoimmune. I think you should have checked his Thyroid for hypothyrodism. It could be Hashimotos.

Considering that patient is 53 y/o he schould be screened for colorectal Ca. Fecal ocult blood test schould be done also, with ofc digital examination, as mentioned above.
Considering that his constipations lasts more that 3 monts and that high fiber diete has't done much good, my goal would be to see was constipation caused by organic disorder or is it due to some other condition.
I would rule out colorectal Ca and then do sigmoidoscopy