If your son has a fever/infection, it can increase his heart rate to 155/minute. It's a physiological response from the body.
Such an increase of heart rate during an infection means that it's a "tachycardia symptom". It's not a true arrhythmia.
So you don't treat this tachycardia symptoms with anti-arrhythmic drugs, you instead treat the underlying cause (here, the sinus infection).
An atrial or a ventricular tachycardia are easily recognizable on an EKG, and the cardiologist can tell you right away (those kind of tachycardia can be life threatening).
If the cardio said that he has to see your son after the sinus infection, then, the tachycardia may not be as serious as you feared of.
If the cardio saw a worrisome pattern on the EKG and/or an abnormality on the echocardiogram, he would have told you right away. He may also had probably hospitalized your grandson.
So you can somehow be relieved. From what I've read from you, it's not something which sends to panic.
A sinus tachycardia is generally not a threat for the body (except in very rare circumstances, unlikely happening to your grandson from what you've said).
Also, if it can completely reassure you, a heart rate caused by a sinus infection does not normally contraindicate to take ADHD medicines.
What contraindicates to take ADHD medicines is if someone has an atrial or ventricular tachycardia. As I told you before, the heart rate does not say whatever it's serious or not by itself, except if it is super high. But at 155/minutes for a 6 years old child, it's not so alarming.
However, we can understand you that you want to rule out a heart issue before resuming ADHD medicines for your grandson.
However, you have to treat a sinus infection, as it can migrate on heart valves (it's called an endocarditis). So, don't let it linger, it can lead to serious, even life threatening consequences.
Also, about tachycardia, just to give you more tools to understand, 155/minutes at rest is very unlikely a sinus tachycardia for an adult (it can happen, but it's exceptional).
For a child of 6 years old, a sinus tachycardia at 155/minutes is not something you need to panic about. I know it impresses and scare.
But it's dangerous for an adult.
So to know what the standards heart rates are for children, you have to rely on children standards : you cannot rely on standards for adults. There are specific standards for children, even a 6 years old child.
Because a child has physiological specificities that don't apply to an adult, and conversely.
A clinical finding has also to rely on age : it's not because it's abnormal for an adult that it's abnormal also for a child (of course, a heart rate at 250/minute is abnormal as for children, as for adults. But a 155/minute at rest is abnormal and very serious for an adult, it does not necessarily holds true for children).
Polyps are tissues mass in the sinuses. It can infect the sinuses, and it may need a surgery to remove them.
So, I repeat myself, but I don't think that it's such a worrisome that you need to panic. I know too well that it's easier said than done, but try not to.