It could be either his blood pressure medication, low blood sugar, or both. I take blood pressure medication myself and it says right on the bottle not to consume it with alcohol. Granted, I've never had any problems, but I also usually only have two drinks when I drink and I rarely drink. However, you mentioned that he hadn't eaten lunch yesterday and was drinking in the evening. Alcohol lowers your blood sugar, so it could be that his blood sugar is dropping.