I suggest 'practising' the MRI.  Ours are always head MRIs, so I cut up a cardboard box and drew the outline of a head and shoulders on it and made him lie on the box while I pretended to be the MRI machine making lots of noise.  Then he has to practice staying super still. 

Is he having contrast or not?  If he is having contrast he will need to have it injected.  In which case I would recommend you ask/tell the nurses to put some numbing cream on the injection/cannula site before they need to use a needle.  If they say 'we have a spray', say 'no thank you, we want the cream'.  The spray doesn't do much at all. 

Not sure if it will help, but I've had to talk children into quite a few procedures like this.  they always say no straight off, but sometimes they come round once they've had it all explained in step by step detail and been able to have all their questions answered etc.  And be honest.  You've probably already done it, but just in case...

E.g. He needs an injection.  Will it hurt?  Well, yes, but like a pinch.  So not to bad and very quick.  He can pinch himself to see what its like. 

For a cannula - Will there be blood?  No, you will look the other way and once it is done you wont be able to see blood. 

For the MRI - will it be noisy?  Yes, they are very noisy, but its just the noise of the machine like a train or car. 

Its much more troublesome when people aren't honest about what will happen or refuse to explain whats going on.  Good luck. 

Thank you very much!We have started playing lying statues!I have also found some videos on YouTube of children having an mri which has really helped. We haven't been told about a dye or need of injection/cannula so can only presume that's not necessary for us!  He is now quite excited about telling everyone!  We shall see!