My Brother Had Open Heart Surgery 2years Ago Because His

My brother had open heart surgery 2years ago because his blood pressure was staying very high. He had 3 blockages. After surgery his blood pressure stayed normal for about 6 weeks and after that his blood pressure is outrageous with numbers like 194/138, 187/140, 200/140. He is on 4 different blood pressure medicines. He has diabetes and neuropathy. What can cause this type of blood pressure? I worry about him all the time. He is only 55 years old.