Use sheets of paper that together are large enough to cover the entire instrument panel. Once in flight sticky tape them so that they cover everything.
Then give the student some attitude holding exercises at a relatively constant IAS eg holding the cowling so it just touches the horizon / some speck on the windscreen directly in front of the guy (use a whiteboard marker to make a dot if you have to). Change power settings & have him/her continue to maintain the attitude.
Do the same with various reference positions. Once pitch/roll skill is developing then include yaw. Still use the same speck but get him to keep it aligned with a significant land mark (as far away as possible eg on the Hz is best)
Select flap combinations & continue the exercise. If manual flap then you extend them but fairly slowly to allow the guy time to perceive & react.
Select some 'typical' attitude/power combinations and have him hold them eg crz S&L, slow S&L, Vx, Vy, Vg etc. Use flap & gear to force pitch changes. Then get him to select those attitudes.
Once the aircraft is stabilised, uncover an appropriate instrument as a
brief reference then cover it again. Uncover it some minutes later to show the general lack of movement.
With the instruments covered have him select those 'typical' attitudes again & once stabilised let him see how the performance has stabilised at/near the expected & pre-briefed figures.
Make sure he is trimming to 'hands off' as soon as the aircraft is stabilised. Ditto for using a forefinger & thumb style of grip.
All the above is best achieved on a very calm day with a good clear horizon. Don't take the fellow up if there isn't one - at least, not until the instrument fixation is 'fixed'.
Use your imagination to give the guy practice at attitude selection, maintenance & control/adjustment. Just make sure his experience progresses from very simple to more complex at a rate he is capable of.
Good luck to you (and him)