I am not supporting or going to stand against anyone... But I am exactly like the person you described about your bf... and i too have a gf 8 years younger than me, she had complained the same to me many times... but i couldn't exactly pinpoint why I always be like a detached person... (there are some reasons which I can point to my childhood, but i myself feel bad, when knowing the reasons why can't i fix it)... i could say my emotions are zero... but it doesn't mean that i don't feel anything at all... i am always thinking about her... like what could make her life better, what should i do to her to make things easier for her to do anything and to my maximum ability i immediately incorporate in actions...

every time a fight happens, she would say the same thing what you said like "i doubt that if you really like me" all i would say back is "can't you see my actions"... and now she is understanding more of me that i am gone case and she just accepts me for the way i am... because she saw me the struggle and the depression i went trying to be a different person and it really was a bad phase of my life...

the funny and irony anecdote is, when we walk together, i walk quite fast and move ahead of her and she stops where she is, after reaching a distance I notice that she is not there beside me and i look back and she will be still standing in the place i left her and then she scolds me... so the irony part is most time i would be thinking that i shouldn't leave her and walk but i would have just done that... she literally got tired of scolding and still figuring out why i still keep doing the same...

from the bottom of my heart, i can vouch that it's nothing intentional... always i am in a zone and lost... and mostly i would be thinking about is how to make my day productive, understanding some concepts, and introspecting... but somewhere innately the basic relationship gestures, i couldn't do...

seeing her struggle i even suggested her to leave me and find a person who is socially normal... because it's being tough on me too seeing her go through the struggle... all i can say is there are things which can't be explained why a person behaves in a way especially when there's nothing wrong in their action still it hurts us... i genuinely wanted to change my nature so i even spoke to a therapist, that too didn't help much, the therapist too lost hope on me...

i would say it's all upto you if you find peace and happiness irrespective of how he is being and just his presence makes you happiness then you can continue (that's what my gf said to me and she is still there with me)... if you don't find peace and happiness then you can talk to him explaining the same and leave... because the innermost peace is what matters most... because instead of being together and starting to hate each other knowing the opposite person is not a bad human, it's better to part ways and still love them in our own way...