Hi,
I work in Radiology and assist with MCU's regularly. The procedure is fairly straight forward, it checks for reflux of urine back up into the kidneys. He will need to be stripped and will lie on an xray table. It's a sterile procedure so the Dr will wipe his bits and be wearing gloves etc. A very small sterile catheter will be passed into his bladder, and secured with tape. Some clear fluid (which shows up on xrays) will be passed through the catheter to fill his bladder and xray pics will be taken when he wee's. As he wee's they will take pictures to see if his wee is going back up the tubes that link his kidneys to his bladder.
The possible outcomes are that he will be all clear and there is no reflux back to his kidneys, or there may be reflux. How serious it is depends on how far the wee refluxes, sometimes it just goes slightly back up, sometimes all the way up and sometimes it floods the kidneys completely. If it does reflux he will probably be referred for a nuclear medicine scan to see if there is any scarring on his kidneys.
He will be unhappy about being stripped, and will not really like being held still on the xray table. We get Mums and Dads to help with as much as we can, as it tends to make for happier babies! He may well cry a bit, but in truth the examinatin doesn't actually hurt, he may feel a slight twinge as the catheter is passed up his willy into his bladder, but once it's in place all he will feel is his bladder getting full, at which point he will emtpy it, generally all over the staff (I always wear gloves and a plastic apron over my lead apron!!) and it's amazing just how far little boys can wee!
As for how to get meds into him, hmm, it's difficult! Is he on trimethaprim? It's the one they tend to use for bladder/water infections, and it tastes vile. Can you mix it with a little milk and get him to take it like that at the beginning of a feed, or perhaps squirt it into the back of his cheek a little at a time. It's hard though, they have the ability to spit it all out don't they? It's a horrible thing to say but often if they are crying they will gulp and swallow, so the meds go in any way.
Good luck for tomorrow, I'm sure you're dreading it but it should be done pretty quickly, and you'll probably find it more distressing that he will.