If he does do badly in the practice tests, when you analyse where he went wrong, why don't you 'big up' the good marks and then quietly practice the poor areas....
In other words, say he gets mainly vocab wrong, talk about how he got X% of the maths questions correct and over the coming days tell him how you don't even have to concentrate on the maths because he did so well, all that sort of stuff.
My daughter came home yesterday with her first test, she did well but it is hardly the real thing and told me that her friend, who is very bright and the eldest child in the family, got a really poor score which has shocked the whole family as they apparently believed looking through the Familiarisation Pack in the summer hols would be fine.
The dad told her she was useless and has let them all down and didn't speak to her at all. She is heart broken. I felt so tempted to phone and ask if she wanted to come round and do some tests with us in the coming days so the girls can work together etc. but at the end of the day, it's not my problem.
My point is, this dad is an oaf (to put it mildy)
but so our kids get the correct vibes about sitting the exam, I think we have probably given them all the academic help we can by now, it's a case of they can or they can't - what will be and all that sort of thing.
Whim-sing lyrical as usual, but good, good luck and 'what will be'