Hello, slight restriction of movement of right eye towards right side is suggestive of sixth cranial nerve palsy. Most common cause for which is idiopathic followed by diabetes. As per your dad's report , you should retest sugar levels as fasting  blood sugar levels are above normal.  You can also check HbA1C( glycated hemoglobin) level to completely rule out diabetes.

VI CN palsy is generally self limiting and I have already told that it will take a few months for it to recover. MRI brain will be reassuring that there is no emergent neuro surgical situation. It is recommended that left (normal) eye is patched to avoid double vision and confusion while doing general activities. Just tape some white paper on the left lens of his spectacles to make it easy. Double vision does not cause any brain damage but is a result of a nerve not functioning right.