How is his vision in that eye? Other than high blood pressure, does he have any other vascular problems (like diabetes, carotid disease, poor circulation)?
With severe redness and pain and presumably decreased vision, there are four things that I would be very concerned about.
One is acute angle closure glaucoma. This where the pressure in the eye goes very high. He needs to be evaluated and treated quickly.
The second is acute inflammation (iritis). This also needs to be treated ASAP.
The third is ocular ischemia. This is similar to a stroke or heart attack where the blood flow is cut off to the eye. This also needs to be treated ASAP.
Finally, but rarely, he may have temporal arteritis.
All of these conditions need immediate attention and treatment.
I hope this helps and thank you.