R..
My DD is 6 months too... been sitting on her own since about 3m, but started rolling tummy to back at around 4m... she still doesn't roll from back to tummy though. I think I have seen her do it twice, so I know she CAN, she just doesn't want to. She has the opposite problem, hates being on her back (unless she's sleeping) and won't take the time to figure out what she has to do. maybe he is in the same rut... he hates tummy time enough that he is so focused on how angry he is that he doesn't concentrate on moving his arm where it's supposed to be, and gets frustrated.