This is likely one of 2 things:
1. You correctly suggested it may somehow be related to a UTI and will be easy to detect through a urine test. It is possible it may be related to crystals that may be forming and is caused by multiple possibilities including diet, genetics, infections etc.
2. There may be a behaviour component - a certain anxiety after the surgery can try and prompt them to get attention in any way in order to alleviate stress or anxiety. A call for the washroom usually gets one's attention quite quickly and many of them (dogs) know this.
Obviously to differentiate, it is best that this man have his boy checked out by his Vet or at least contact his Vet for advice. If he feels he is not getting adequate support, he should look for another opinion. If it is not a urine infection, I have found that reverting back to crate training for a while may sometimes help
Hope this helps
Martin Slome DVM
Centre Street Animal Hospital
7700 Bathurst Street #40-42
Tel. 905 771-9855