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. I've been told by a doctor that I have hypoglycemia, though, not officially tested, but if my blood sugar gets low I can feel it in my body. I will get very shaky and just feel "gross" inside, for lack of a better term. The fact that he feels better later on leads me to believe the alcohol is the culprit, but not sure if it's interfering with the BP meds or blood sugar. Next time he has some drinks I'd make sure he's eaten regular meals and if it doesn't happen again, it's likely that his blood sugar has been too low recently.
could be he is allergic to alcohol and you should never drink while taking meds
Could be his blood sugar as well, have him get the Dr. to check him ,he could have low blood sugar since he doesn't eat much before drinking this could cause one to feel like he does.