Talk to the doctor, and also ask the psychologist ahead of time what they want to see - the 'baseline' level of the kid 'au naturel' or how he is at his best, when medicated. If the psychologist wants to do some testing, to see what his potential is, and he functions better on the medications, then clearly it would be best to medicate him. It really does depend on what the psychologist wants.
My vote would be to medicate him as usual, but for us at least, we are given some leeway in dosing our kids, so we can adapt to any one-off change in their routine. An evening exam, for example, would benefit from a late afternoon dose (we've been authorised to do this). The drawback for us - he finds it harder to get to sleep, so of course we don't do this very often.
Don't vary medications without permission, but if you ARE given permission and the psychologist wants at least a glimpse of your son unmedicated, then take a small dose with you to give him once the "Exhibit A" situation is behind you. But again - check with the doctor, and also make sure it's not too late in his day for him.
We tend to carry spare supplies anyway. We've had a few unpleasant times when an early start has meant that kids were unmedicated. easy child 2/difficult child 2 totally blew an audition out of the water once, by forgetting her medications. Not good. She was angry with herself, because it would have been a really lucrative gig if she'd got it.
Marg