Abnormally fast heart beats can be dangerous if left untreated for too long. You should take his pulse, here is how. Find his pulse in his wrist, and count how many beats you get in 15 seconds. Multiply this number by 4 and that is his heart rate
. If it is in the nineties, or up and he's not doing anything, it could be a blood pressure problem. But dont get to freaked out, he may just have a faster heart rate, than others. If you are truly worried about it, take him to the doctor and try and get him referred to a cardiologist (heart doctor).