He needs to have an EKG. It sounds like he's having a rhythm problem, but I'm not a doc, so don't take my word for it.
I've had arrhythmias for 17 years and that fluttering is what I feel when I am in atrial fib or atrial flutter
. It sounds like he is having a vagal trigger response. It happens after a big meal or lays down. I get it when I lay on my left side.
Just so you know, the atrial arrhythmias are more annoying than dangerous. I know that anything involving the heart can be scary, but since he'll be getting checked tomorrow - try not to worry.