Has he been diagonsed with OCD or are adults just observing symptoms? It is possible that if he has sensory issues they are making him have behaviors that mimic OCD but he doesn't really have that disorder. I found that out because as a toddler & early preschooler we thought our son had OCD, we even had him tested and I have a piece of paper saying the specialist observed OCD tendencies upon evaluation (around his fourth birthday.) Soon after, though, he started occupational therapy for sensory integration dysfunction and most of his obsessive behaviors had ended or lessened significantly within a year or two. A child behaviorist told me that the sensory issues can make them behave like that but it's not always true OCD, the child just feels so out of control in their body that they are trying to establish order and control externally.
Begin by contacting your grandson's family doctor or pediatrician. He/she should be able to give help with this disorder. If not, then ask for a referral to a specialist with experience in anxiety disorders in children. Three years old is not too young to have this issue addressed.