
this is typicalof a dog that has had Obedience training first, he has probably learnt that if in doubt sit, and you are sure to be rewarded. He needs to learn that Stand is different, I would step back when he is about to sit saying Stand. If it is when the judge is going over him, place your hand on his stifle (preventing it bending0 and keep repeating stand. If he were a bitch I would place one hand under the tummy stroking to keep her up!
In my breed we free stand for a tit bit, so moving him as soon as he sits, with titbit held so that he would have to stand to reach, held behind you, should do the trick. after he knows stand I would progress to the Stacking, holding under chin and tail.
It just needs some patience, do not get frustrated, as when unsure he will revert to any behaviour that normally gains approval, ie sit!
Barbara and the Grey Curly Tails.