It is too small a sample but it’s the only sample we have. It’s also a damn sight bigger than a sample of one cherry picked game. If you want a bigger sample you can add the games he played last year. It starts to give a picture of his ability in that grade, which for his age is a decent predictor of what type of first grader he’ll be.
But also in first grade he has been very much second/third/fourth fiddle, so his line engagements and runs are much lower than they are in reserve grade. In that grade he gets the ball when the defence is out of shape, whereas in first grade it’s a waste to give those touches to the backup so he rightly doesn’t get those opportunities to shine that he does in the lower grade.