Any advice/experience on UTI in an elderly male with indwelling catheter?

The catheter is probably causing the problem. Better to prevent than to wait till he has one. Of course the catheter needs to be kept clean. As said, water helps to flush the system. My Mom was on cranberry tablets and a probiotic for the last year of her life and not once had a UTI.

Alva, a member of the forum and a retired RN, swears by D-Mannose for prevention. You can read up on that.

My late husband had a supra pubic catheter and occasionally he would get a UTI. His always started with him having a fever, as that is common as the body is trying to fight the infection. Do you take his temperature regularly? If not it might be a good habit to get into.
His urine might also have a stronger smell to it when you're emptying his catheter bag, so pay attention to that.
Most importantly make sure he is drinking plenty of fluids during the day. Water is always best, but my husband usually drank the sugar free Powerade. Also give him foods that have a lot of water in them like watermelon, cucumbers, and the like.
It's hard I know, but just keep doing the best that you can.

Prevention:
Drink lots of water. Every day. Your dad can drink other things, too. But he must drink lots of water.

Examples of early warning signs UTI:
…Change in the color of the urine
…Murky? Clouded?
…Change in the quantity of urine

I think your dad has a urine bag? The urine goes from the catheter to the bag? That bag can be monitored (look at the urine).