Welcome to the board of Health Care Magic.
As you have written the symptoms, sever headache, falling asleep and collapsed some time.
Possibilities are ;
Your brother in law might be developing some space occupying lesion/any growth etc
as since he is insulin
dependent diabetic. He might be taking some extra insulin which may be causing chronic hypoglycemia
which in turn may be, may be the cause of excessive sleepiness and some brain infarction
( brain dryness). which is visible in the MRI
Hope your query is answered.