There can be various reasons for recurrent UTI in your father and the cause must be established and then a proper treatment given.
1.. Uncontrolled diabetes
results in lowering of immunity in the person and also the sugar in urine makes it a favourable place for bacteria to grow .
2.. There could be obstruction to the urine like kidney stones or prostate
enlargement.This obstruction impedes the flow of urine making it more susceptible for infection.
Chronic prostate infections are difficult to treat because antibiotics are unable to treat infected prostate tissue effectively.Men with PROSTATITIS
often require long term treatment by a carefully selected antibiotic.
Do the following....
Control the blood sugar levels meticulously.
Run tests like IVP,ULTRASOUND
etc for finding any obstruction.
Get urine culture and sensitivity done and start a proper antibiotic therapy for a long time.
In case any obstruction is found,that should be treated.
I hope it helps.