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Diabetes can cause slowing down of gut and constipation. If there is hypothyroidism associated then it can aggravate the situation. Chronic constipation can also be due to diet poor in roughage or fiber, dehydration, colon disorders, irritable bowel syndrome, diverticular diseases, cancers, severe acidity etc. It can also be due to milk allergy and other food allergies. So first his diabetes status needs to be checked. If your boyfriend is on any medication then ask his doctor if that is the cause of constipation.
If both these factors are ruled out, then other causes of constipation will need to be investigated. Please consult your PCP regarding this as a confirmed diagnosis is not possible on net. Hope this helps. Take care!