He has bloody stool and the vet says hairballs?
Did the vet actually examine the stool, or is he guessing? Lots of vets do not examine stool, I know from experience.
Several thoughts --- new vet, pumpkin added to food, Metamucil sprinkled on food, water added to food, fresh water put out more often or a fountain (cats are weird about water sources - you need to entice them to drink more)
Blood in stool is a worrisome sign. It could be nothing, or a host of things. If your cat is allowed to suffer constipation too long, he could get megacolon. That would be a disaster for both of you.