He is insecure, I am sure you have assured him that you are not doing anything but he will continue to ask and it is going to ruin your relationship, or as they say the one accusing maybe doing something and trying to argue with you, you never can tell. Sit him down and just explain how much you care about him and you wouldn't do anything to hurt him but also tell him he has to stop accusing you because like I said above you will get tired of it and always argue and then the relationship will be over, because you couldn't take it anymore. Good luck