I have very bad ocd and have had it for 23years. Ocd is an anxiety disorder and it will not get better on it own unless he sees a therapist or gets on meds and sometimes the combination of both. But I think therapy is always necessary. The best thing you can do for him is to not entertain his obsessions. You should let him know you are there for him and will comfort him but do not offer any reassurance to him. Depending on what his main theme is with ocd like lets say he has harm ocd where he is afraid he is going to harm someone dont try to reassure him and say " no you wont" just ignore his attempt to seek reassurance and only enforce the fact that he shoould be accepting everything he fears to be a possibility. When people with ocd learn to accept their fears as something that could possibly be true no matter how crazy it is then they will have to learn to sit with the anxiety that comes along with the thought. Once they have done that so many times they will being to desensitize themselfs to it til one they they will have the same fear come up again and will no longer be afraid of it. This going for all obsession even if he has a obsession with cleanliness you try to make sure there are tiny messes around the house, within reason, so that he has to be forced to face it everyday and learn not to fear it. I hope I have been able to give you some good advice.
you have already received some very excellent advice. Don't try to simply point out reality in order to disprove his fears. The best thing you can do for him and yourself is pressure him to get help with a professional therapist. He cannot do it alone.