Does he have high blood pressure and/or unhealthy blood sugar levels. Either of these would be the most likely culprit. Another cause could be a heart murmur or irregular heartbeat. Inappropriate blood sugar levels can also cause irregular heartbeat and a "fluttering" feeling of the heart.
One time, at school, I had not eaten anything all day and had not slept either, I drank a Mountain Dew which has a good amount of caffeine and sugar, which caused me to feel my heart kind of "flutter", or beat very irregularly and sometimes skip beats. I started tingling and going numb, so I went for a walk and it eventually went away.
My point is that any of those conditions are quite serious, and if they are recurring, they need medical attention. He should get a physical and a blood test, and he should tell the doctor what the problem is.