OOoh, we've been through exaactly the same and I was just about to post a question about it!
DS (now 8 months) has a severe dilation on his right kidney which was picked up in utero. He's had scans every month since he was a day old, and he has the kindey function test, and the test for reflux when he was around 6 months.
The catheter bit is fine - they cry a little bit, but the area is numbed a little first, so they don't really feel too much. Once the catheter was in DS was fine. For the kidney function test (I think that's the nuclear renal scan) they put him in this blanket with air in, then sucked out the air so that he was held very still. He even fell asleep during the test, I think because he was so warm and snug in there! The worst bit with that one was when they put the IV in, which did make DS cry a lot for a few minutes.
Honestly though, they soon forget about the test. DS was fine once he'd had a bottle and a sleep afterwards, and even went to creche in the afternoon.
he needs to be well hydrated for the renal scan - they told me he had to have a bottle beforehand. And they like you to feed them after the scan too, as they have to drain the contrast material from the kidney, and also need to see how well the kidney drains.
Hope it all goes OK for you/your DS. I'm sure he'll be fine with all the tests - it's amazing how well they cope with these things.