I know some about body language, but what I know mostly is that it always depends on the context. When a person has his arms folded infront of his chest this could be a sign of wanting to protect himself from the conversation. But without the context we don't know. How are his feet? Does he nod and actively engage in the conversation? Maybe this is his favorite way to sit and has nothing to do with the -current- situation. Maybe he is cold.

So, I would not try and judge your T's involvement based on one part of his body language. I do think that you noticing this and worrying over it to the point of making a post about it is enough reason to talk to him. You feel that he is not engaged in your therapy it seems, so that is something you should discuss.