You have just got this boy, who I know is a driving horses, trained, home.
You have said yourself that you want to get him used to
your voice and commands before driving him, and this is sensible and sensitive of you to think of him!
So, why are you surprised he is not obeying you?
Think about it- you answered your own question- he does not know you!
He has not forgotten how to do these things, he just does not know your voice and he is in a strange place and he does not know any of it and, by jingo, he odes not see why he should so there!!
Equine equivalent of a (IMO rather justified) cross between a very low key hissy fit and total bewilderment.
Can you round pen, as opposed to lunging?
I ask because I really do like a horse to have a choice in a situation like this, to be able to make up his own mind.
If you cannot round pen him loose, then give him a loose rein and just keep on repeating the command (I would start with him in his free canter and work on bringing him back to a trot as
stopping a horse, especially a driving horse, is SO much more use than being able to make it go
) and just keep on until the light dawns- and it will, trust me.
He is probably still a bit disorientated, too, so I can see you are already cutting him some slack- be a bit firmer now, ask nicely, and if that does not work try a hard "Whoa, stop messing me aroundyou'orriblelittle'orsedon'tmakemecomeoutthereandgetafteryou....." (you get the idea-I tend to say silly things to my horses because externalising my feelings helps get the general idea across!
Honestly, I think he is just confuzzled and feeling maybe a bit stroppy, and add a pinch of "let's see how far we can push her" and stir well.
You are on the right track, just keep going a bit further, that's all.
Good Luck....
Where are those flipping pictures?????