I,ve tried to broach the subject with him casually a few times but he kinda just goes akward / pulls a face / change the subject..

Maybe I should sit down and have a bit more of a structured conversation?

My BF has spoken to him about what he would be comfortable calling me but with much of the same response. Although BF maintains that this is just the way he is, especially if it someone new in his "family" which he thinks is actually a good thing as he sees me as some sort of "family". (He took 2 years to assign some random nickname to his mothers boyfriend)

So one the one side its how it makes me feel (like a nobody, disrespected & unapreciated) but on the other side it makes me worry about the kid, why is it so difficult for him, does this point to some emotional / social problem. Did his parents split cause some dissociation?