I read your question and I understand your concern.
High blood pressure can and should absolutely be treated or sooner or later there will be serious consequences. You don't mention the numbers of those very high values, but the blood thing indicates nosebleed due to very high pressure causing the vessels in the nose to rupture.
Regarding the cause, there are cases of
essential hypertension, meaning no cause can be identified, but there are cases especially if manifested at a young age when a cause can be found and be treated like
kidney conditions, abnormalities of blood vessels, endocrine (
hormonal) issues. Those are identified by appropriate imaging and lab tests. Even if no cause is found there are many drugs available to control his blood pressure.
Of course first he must understand that his ignoring the problem could be serious, that the condition is treatable and that he must be evaluated by a doctor as soon as possible.
I hope things work out for the best