This is an interesting question and I recommend that he speaks with his doctor
about it. In general, straining during a bowel movement tends to increase the blood pressure in the top of the body. It is known as a valsalva maneuver, or straining against a closed glottis. Because it increases blood pressure in the head, it can be a time when people are more prone to passing out or having other cerebral events, or even vascular events. This is why (or at least one of the reasons) it is not uncommon for people to pass out in their bathrooms. When you consider the effects of a valsalva maneuver, it is easy to see why some people will have headaches or feel that increased pressure in their heads. The interesting thing in this situation is that your father can actually feel that his head is more tender. This makes me wonder if there may be something on the top of his head that is more prone to engorging with blood when he increases his blood pressure. While it would be unusual, there are some tumors, both benign and otherwise, that could preferentially be sensitive to these sorts of changes in blood pressure. Please have him speak with his doctor about this question.