Hi, J.. Mention it to your pediatrician of course, but don't panic - especially if his coloring is okay (rosy cheeks, etc...).
My son was born with something called sinus arrhythmia and it used to REALLY freak us out when he had a fever as an infant (he was my one kid who got ear infections every time he cut a new tooth and the fevers would - no lie - get as high as 105 or more). Fast asleep, high fever, I'd listen to his chest and the "off beat" business was even MORE dramatic than ever; to the point where it would go, "tick-tick-tick-k'chooooong..... *silence* ....tick tick..." that sorta thing.
I went as far as having him checked by a pediatric cardiologist for a few years (I have a mitral valve prolapse, same with my mother - this just made me feel safer to have these checkups for him). The long and short of it is it's pretty harmless. Something I was always told he'd grow out of when he hit puberty.
Sure enough, he's almost 13 now, I hug him after he's been running outside, I listen..... can't find that "off beat" business anymore. Don't worry too much, hon, just definitely mention it next time you see the doc. Take care.