Boyfriend afraid of long term relationships...

So my boyfriend recently told me he's scared of being in a long term relationship again because of how his last relationship went down. His ex went crazy by the end basically so I do understand why he's unsure if long term relationships actually work out. We've been together 6 months now and things are going really well. It's not like he wants to break up with me but I feel like he's waiting for us to fall apart and start fighting or for me to turn into his ex. I want him to feel secure and safe with me. I'm not his ex. And I'm not going to turn into some kind of overbearing and controlling monster. Does anyone have any advice on how to handle this? How do I help him get past this fear?