What Causes Fluctuations In Blood Pressure Along With Dizzy Spells?

Father in-law is 82 years old, COPD, 1 stent, sugar is being tested 4 times a day usually being very low or very high requiring insulin to try to stabilize. Normally now stays lying down or reclined, as sitting upright or trying to stand tends to cause dizziness and increasing blood pressure. On few occasions has passed out attempting to walk after rising up. On medications for prostrate, blood pressure and depression. Currently in rehab facility for physical therapy which he cannot do. We cannot seem to get an answer as to why he continues to have the dizziness spells and blood pressure fluctuations. Any info or insight would be appreciated.