Diabetics are prone to infections and this is very common in those individuals in whom the blood sugar is not controlled. It is also very difficult to eliminate such infections in diabetics and it requires prolonged therapy with IV antibiotics (which he is getting).
Another problem which might predispose him to UTI's (at his age) is prostate
), which can lead to obstruction and stasis of urine - which then predisposes to infection. It is important to get this investigated by an urologist
You should also consult your endocrinologist
and make sure that his blood sugar levels are under controls.
Hope he gets well soon!