Your dad is a very elderly person and is having diabetes
for pretty long time.
He is also suffering from Diabetic nephropathy
for which he is undergoing dialysis
Now he has loss of appetite and he sleeps extensively.
His elderly age, diabetic stage and his renal function all contributes to his problem.
Because he is not taking enough energy into his system he is tired and sleepy.
Frequent small easily digestible meal is advised.
Keep a watch on his fluid intake.