[Meursault]

scores of books have been written on interpreting body language, and they would all agree with you. there is a point at which inattentive "turning away", besides being a sign of either problems within him or problems between you two, becomes simply rude (especially if he is conscious of it). when? i think the only answer to that is the very one you pointed out --"cues any normal person would take badly".
but what about the context of his behavior? if he habitually 'turns away' from everyone then the meaning is a lot less you-specific. i once had a roomate who had a hard time looking anyone in the eyes for long, and would, habitually, physically face away from people he was talking with. over the long run it became obvious he had deep problems and he ended up being institutionalised. the inability to face people was probably a manifestation of this.
then again your bf's 'turning away' may indeed be you-specific. even then there are a number of possible 'meanings'. and again, context might tell you something. does he turn away when you start talking about 'relationship stuff'? does it coincide with criticism or jealous questions?
then there's the possibility that the body language is part of a larger pattern of manipulation that could be considered emotionally abusive. dysfunctional people like leaving things in doubt, and other dysfunctional people assume the worst and react with clinginess. (they make grrrreat couples lol.)
there are simply a lot of possibilities, and they all depend on Context.

only you are actually privy to that context. it would be illogical for any of us to conclude anything based on slim information, and irrational for you to trust the conclusions of persons willing to make conclusions on slim information.

anyway... what i don't recall ever seeing is a book on how to modify another's body language. there are only two things i can think of trying. the first is to just to use open language yourself and see if he eventually follows. the second is to mirror him, but without letting it turn into an exercise in passive-agressive bullshit.
but as you pointed out yourself, body language is only a reflection of something in someone's head, so only addressing body language is unlikely to open things up. it is simply a necessary condition, but not sufficient of itself.
and here we reach a territory i won't enter, both because the number of possible paths is huge, and because they are not paths i travel. perhaps you should be trying to create desire, or trying to assuage some unspoken fear, or balance some mental account left in the red by a previous gf. whatever the answer is, body language will be only one of your tools. and whatever the answer is, it's a question of sales psychology, and not one i ever bother with. when i find myself dealing with a consistently 'closed' person, i just accept that as the limit of the relationship, and if i want more i look elsewhere. yeah, that can be disappointing, but there are tons more fish in this sea.

so, um, sorry if this isnt much help. but it cant be lol. good luck tho