C.K.
I agree 100% with everything Kristin has said.
I will also add that it's possible that he is in pain when you are rolling him over or doing the sit-to-stand. Unless you've gotten instructions or the "OK" from a healthcare provider, I would not be putting him in any physical positions that get this reaction. My son was 9 months and could not sit unsupported and would cry when we would sit him down. We brought him to a pediatric chiropractor, and his left hip was totally out of whack.
What does your healthcare provider say about his development? You might be worrying for nothing.
I mean no disrespect by what I say next, but it's just my opinion... Outside of it potentially being a pain issue, rearing the head back is a normal phase most babies go through. It's not like they have a lot of options on how to express themselves. I never took it personally or let it bother me when my son did it. Really--"This too shall pass". When he is a teenager, you will look back fondly at this time, when your biggest issue w/him was rearing his head back. ;-)