I would ask another Vet. I know that may say silly. I would also do a search on the Web for answers.

I know that cranberry juice is often given to humans to deter UTI infections and they come in tablets too.

If I were you, I would try to clear the infection one more time, then really try to get more water into him, lots of exercise and a strick strick diet of special food.

Cutting off his penis sounds so drastic to me. I don't know what it entails, but it just sounds awful.

Gosh. Whatever you two decide will be fine, I am sure, but gosh, what a decision to make!

Is there a Vet School somewhere nearby? Can you contact them?

Keep us posted.