Dog peeing blood for a few months

My dog peed a fair amount of blood a few months ago. He had bladder stones and when the vet xrayed him after the blood it showed that he had 1 or 2 stones left. Seems like he had probably passed the stones and this was the cause of the blood. It's been a few months now and he is still peeing blood. Not all the time but every so many days. He is acting normal and eating normal. The vet has put him on Urinary SO hard food since the fall and he is still on it. Any ideas what could be the cause of the blood? I have made an appointment to see his vet in the next few days. Help!

Prostate problems can cause a dog to pee blood, maybe ask them to check his prostate, its a 2 minute physical examination.