If there is just a tiny amount of blood in the urine, the larger the amount of urine there is, the less likely it will look pink or show blood. The smaller amount would show more blood because it's not diluted as much.
He may have another urinary infection and if so, it's possible there is something else causing it. Perhaps he has a bladder or kidney stone or crystals in the urine, all which irritate the urinary tract and can cause infection. The vet can check his urine for crystals and bacteria. Since he'es a boy, it doesn't necessarily have to be drawn directly from the bladder so if he pees on a sterile surface, it can be sucked up with a syringe.
If not already doing so, consider adding a urinary health food to his diet. These foods change the PH of the urine which makes it a more inhospitable place for bacteria to grow.
Hopefully it's just a UTI.