Many a times patients with diabetes
are detected incidentally and they do not exhibit any symptoms at all. It is very common. His acute infected status might be the cause of his high sugars but that means any stress
would have made the diagnosis apparent in the future.
So take it positively and follow the advice your health care provider would have given to him and you as family member.
If you get his HbA1c
test done, and if it is more than 6 then it means he has been running with high sugars since more than 3 months but it's just the infection which exposed the diabetes now.
I hope you got your answer. For any related query get back to me. Take care.