Just some tips for the MRE... and someone please correct me if I'm off on some things...
My son was 11 when he had his first MRE and drinking three glasses of the contrast was tough. They do flavor it for kids, but after awhile it just tastes yucky. We had a Child Life person working with us and we just let him drink as much as he could initially. After that we watched a clock and at the 45 mark, she'd have him drink for 10 seconds. Then could take a break, walk a few laps around the receptionists desk (to get the contrast moving in his system) and then come back. It would be close to the 45 mark again and he'd repeat. Try to make a game out of it if you can with distractions and little challenges. Make him walk backwards one time or do the lap on a rolling chair. This is a tough age where they don't necessarily want to be treated like a little kid with distractions, yet they still kind of need it.
In the end my son only managed two of the three glasses but by then he was in tears and super nauseous so they let him stop. The results weren't ideal since he didn't get enough contrast in.
While doing the actual MRE, even though he was allowed headphones to listen to music, he had to be very active. Hold your breathe, move this way, exhale, hold your breathe. It was quite the work out and took a good hour. He was exhausted afterward.
Last, be sure to have him drink lots and lots and lots of water afterward to flush the contrast out of his system. He may be nauseous or have a stomach ache for a bit, but keep pushing the water. It's very important to get the contrast flushed out.
Let us know how things turn out and good luck!