I have to have an MRI every year and am extremely claustrophobic. Luckily it won't be too long for just the brain.
The number one thing that helps me is I ask the nurse or second radiographer to hold my hand or leg while I'm in the machine. It seems trivial but helps me feel more 'connected' to the world outside and not as claustrophobic. I imagine for a child you would be allowed to be present in the room and I think that would help a lot.
In some machines the thing they put over your head has a mirror that acts as a kind of periscope to see out. This helps me. If this isn't present though I absolutely do not open my eyes as seeing the enclosed space makes me panic. I would suggest he decides on eyes open or closed beforehand, and if he wants them closed then maybe close them before the 'head cage' thing is put on. I believe lots of places have the tech to play films etc, so this might be an option for a child. I always accept the offer of music if they suggest it.
It sounds silly, but if he's up for it I would recommend cutting a window in a cardboard box and having him lie down in bed with a pillow under his knees, earplugs in, pillows on the sides of his head, for 15-20 minutes while you play a Youtube video of MRI sounds. It will simulate the actual situation pretty well honestly and show him that he can handle it for the amount of time that's needed.
I would probably give him some sort of placebo herbal remedy beforehand. Personally I try and go to the appointment feeling tired if possible.
It's highly likely he will actually find it okay. I wind myself up before every appointment, remembering traumatic long brain and spine MRIs, but really the brain one often only lasts 10 mins. The radiographers have been without exception lovely and kind so I'm sure they'll put your son at ease.