We went through a whole battery of blood tests and x-rays with my youngest. Assuming you will be doing this at a children's hospital, the phlebotomists who draw blood are usually very good with children. That being said, our experience was difficult anyway because they did have to draw quite a bit of blood (like 11 separate vials of varying amounts). They put the needle in once, and then just change vials to get the blood they need for the various tests. For my daughter (she had just turned 4 but weighed only about 30 pounds) it seemed like kind of a lot of blood for such a little person to give! My advice would be to bring something along that your son really likes, such as a Nintendo DS, or maybe a new game on your iPhone, or something like that, and try to distract him before they bring the needle out. I didn't do that, and my daughter FREAKED when they had to insert the needle. It took me and 2 techs to hold her down. She fought and screamed. :( She has always been really afraid of needles, though. Afterward, she didn't even have a bruise, so I think they did a really good job, she was just so upset by the sight of a needle going into her arm.
Also, you are totally right in your idea to try to schedule the x-rays first. That is not a scary process, and in our case, the tech took the time to show my daughter some x-rays, and showed her how the "big giant camera" (x-ray machine) worked. That went fine and my daughter liked it. BUT, originally I'd had it scheduled right after the blood draw, and she was too upset at that point to have the x-rays done, so we had to cancel and re-schedule for another day. Ugh!
I hope your son's tests go well and that they find everything is normal and he's healthy!