Okay, this probably isn't a big deal.
First... heart attacks TYPICALLY are quite painful. Lots of pressure.
Second... if he is not incapacitated (e.g. can't walk), the severity isn't an emergency... most likely.
Third... I'd guess that he has an mild arrhythmia
(e.g. atrial fibrilation); however, he's pretty young to be having that type of problem. Although, it's not unheard of.
I'd guess he's having a premature atrial contraction
. Also known as a skipped heart beat. There are other more serious premature contractions such as ventricular (which is really really really bad), but if he's not having other difficulties while this happens...
Pretty much everyone has an occasional skipped heart beat. The heart recovers its normal rhythm. The majority of people don't even feel/notice the skipped beat. Because your boyfriend does, to be safe, he should see a doctor.
Be aware that if the problem is not occurring at the moment you're visiting the doctor, he'll ask some questions and that might be it. Occasionally, they'll set people up with a portable heart monitor to record activity for 24 or more hour period. You'll have to record significant events such as strenuous activity so that there is an explanation for any changes in rhythm.
Don't sweat it too much. Stress
can cause abnormal heart rhythms. Lack of sleep, too. Too much caffeine, too.