I'm curious how you noticed the heart rate change? Is he complaining of feeling strange or chest pain? If his heart rate increases a bit with an inhaled breath and slows when he exhales, that's pretty normal. Our normal sinus rhythm will speed up and slow down in relation to what we're doing - sleeping, walking, moving our arms around.
If his heart rate is jumping from slow to very fast, it would be good to mention it to his doctor.
Hi Did you ever find out what was up with your 3 year old, my four year old has the same thing I noticed becuase I was checking his pulse when he was sleeping, he has such a strong pulse compared to my 10 year old, seems to be normal then speed up then normal again? so wierd, the dr did say he has a mild heart murmur a few years ago when he was sick with bronchitis, but then other drs say the cant hear it, some say they can ?!? wondering if I should take hime to see the dr and get and ECG, as I am freaked out.