A 6 y/o can as far as I know never have high blood pressure without a reason. So-called essential hypertension or primary hypertension (which is adults is the most common, 95% of all cases of hypertension) often have a number of reasons, most of them have something to do with an unhealthy lifestyle.
For example, some of the following factors: Stress, smoking, alcohol, overweight, lack of exercise, heavy salt intake, high fat / unhealth carbs intake, lack of sleep, too much work, anxiety, in general all the things we shouldn't do. This rarely happens in a 6 y/o.
Even a blood pressure of 180/95 shouldn't cause severe symptoms, but one of the following two conditions can be likely:
1. He has got a (unknown) condition which is causing pain, discomfort and stress (as well as the other symptoms), also increasing blood pressure.
2. He has got a secondary reason for high blood pressure. Excess cortisol or stress hormones (can happen from a variety of reasons), narrowed kidney arteries or another reason.
I'm not a doctor. I would STRONGLY ADVICE you to see another doctor regarding his condition. A blood pressure of 180/95 is not normal in a 6 y/o (or any other person) and you should not accept the diagnosis of "hypertension without a reason". There must be a reason.