Maintaining a relationship with partner

Hi,

My boyfriend got diagnosed with Asperger's about 2-3 years ago and he is still finding it difficult to deal with some things.  We have been in a relationship for roughly 18 months, but we was really good friends for about 2 years before. It's a very healthy relationship as we don't really argue, lately he has been finding himself becoming unhappy in himself - he has always said how he feels like he is holding me back and that he worries that he cannot give me what I am looking for especially in the future and that I am "too young to deal with all of this" (that phrase really annoys me). I am constantly reassuring him that I am happy and he is providing me with everything I need in a relationship but for some reason he cannot get the thoughts out of his head. 

He is always telling me how I am his best friend and that we're perfect for each other but he is not sure if he can function in a long-term relationship. (He has had a 3 year relationship before me which was very toxic as the girl was manipulative and abusive). Since this whole lock down, his depression is worse and he is finding it hard coming to terms that he is the only person that has been furloughed in his company. He keeps having breakdowns about how unhappy he is and that he thinks it'll lead to a breakup which it nearly did last night. 

I just don't know what more I can do to help him, we love each other very much and don't want to lose each other but I'm scared he will end things and then regret he decision. 

If anyone has any tips on how I could try to salvage our relationship and maintain a long relationship and help he get rid of these thoughts would be appreciated.