How do I make my man feel secure?

We have been dating for more than 3 years now and it’s getting a bit rough. He genuinely thinks i don’t love him and idk why I mean I have done everything that a good Partner would. I have spend many nights reassuring him. I am a bit taller than him and fair skinned and he is darker skinned and just a tad bit shorter which is his biggest insecurity. He has this Belief that Every time i step out every Man eyes me and everyone is falling in love with me I know it’s ridiculous I tried to talk to him about it Im like hey Yk most people usually mind their own business and no just Cz I am a fair skinned female does not mean Suddenly every guy is head over heels for me. He genuinely believes I am gonna leave him for someone else.

He isn’t toxic he treats me really good and is incredibly loyal. But the problem is He is suffering from some mental health issues ever since his father passed away. He has serious abandonment issues and it’s been hitting him harder now that Im in university he keeps feeling like I have a “side chick” in uni for whatever reason. And he has been really sensitive i mean mid convo he gets triggered and then he is fully Convinced that I have changed and am planning on leaving him and then again he apologises and stuff. His mental health is really bad to the point where he had to go to the doctor he would “blackout” randomly but he said after I came to his life those slowly went away and he would also have intense memory loss and idk why but they are slowly coming back he is becoming more forgetful more sensitive and traumatised.

I wanna be there for him through everything he really loves me and is Perfect it’s just I wish I could make him believe that he is fine that there’s nothing wrong with him and im not gonna leave him. I have proved it through my actions already by actually being there for him but idk what else I can do. How do I Make someone with abandonment issues feel safe and secure with me? How do I make him believe that no I am not gonna leave him or cheat on him? How do I explain to him that me stepping out of my house doesn’t automatically mean all men are Targeting me? (Ik all of this sounds incredibly silly)

So what should I do? I really wanna understand what goes on in his mind and whats really bothering him and whats the core issue and how can I fix it