It's only his opinion that the others had made up their minds, unless they talked about it, and he's not on the jury.
So if he's asked and said yeah we talked about it, that's a mistrial. But if it's just his opinion, no.
Also, I think most jurors are at least definitely leaning one way or the other once both sides rest, because they've heard everything. And as long as they're willing to listen to other jurors once in the juror room and be willing to be swayed, that's ok too.
On my other jury duty case (robbery with two defendants), I was pretty convinced for guilty for the 2nd defendant heading into the juror room, and was convinced to change my mind by the rational discussion in the room. The case against the 2nd was really really weak, and the victim (I think) lied on the stand when describing the 2nd robber, because his description of the 2nd robber was different on the stand than the one he gave the officer at the time of the crime.
