At one stage in my life I was very firmly convinced that I'd be doing my partner a big favor by ending the relationship, and that they could go on to a better and happier life with someone else. As the time intimacy was about zero. I had no libido at all.
This was all caused by a number of psychological factors, including depression. I had come to believe I was the cause of everything being wrong in my life, not just my relationship.
Later I came to see that this was simply flat out wrong. People do not get over relationships and switch to someone else just like that. My views on my self worth were not real, they were depression thinking for me. As I improved my desire to remain with my partner came back, and my libido improved most satisfactorily.
My partner remained with me, a testament to love and strenght, and I'll be forever grateful I was not abandoned and could count on their permanent presence - it meant a lot even when I did not realise it.
You BF may be in a similar position, and lack of libido may have lot to do with it -it does for a number of males. This of course, as you would know, could be just the way a person is, though just as likely can come about for a variety of physical causes and as you can see above from mental health issues too.
What you do abut it I'm not sure. Perhaps one avenue might be to persuade your BF to have a thorough physical exam, and at the same time extend it to a mental health assessment too.
Do you think this might be an idea?