There is nothing you can do but trust his word if there is no proof, and he doesn't show that kind of behavior. At least he is made aware of the accusations and is having good communication with you about it. For me I would be more cautious about his ex, with her history of mental illness. Don't let her interrupt your life...block delete, set your SM to private/no IM...if things do escalate, I would be exiting the relationship.