Male cat has crystals in urine and peeing in house.?

We have a male cat that started peeing different places in our house at 6 months. We had a vet heck him and he found that he has crystals but no stones. He recommended prescription diet food, but we were using the purina brand for urinary health. He still pees in the house every once in awhile. His he peeing in the house because he still has crystals? We just want to make sure that its not a behavioral problem. Thanks