First of all, I am sorry that you are having to deal with this! I took my daughter (when she was less than 3 months old) to the emergency room when she contracted a nasty cold. We were traveling at the time for her first christmas and would not return home for a few weeks, so this is why I took her to the ER to be seen. They did an X-Ray on her to make sure she had no fluid in her chest and it was Terrible! The process involved stripping her nude and placing her into some kind of Chinese torture device for infants that keeps them bound in an awkward positon, long enough to do the X-Ray. My instincts told me that she was going to fine, and did not need the X-Ray. The X-Ray of course, showed everyhting to be just fine. Looking back...I would NEVER do it again.
Second story, my friend's two year old just recently needed dental work. It was a similar senerio where her appt. was at 8 AM and she was not to have anything after 12 midnight. My freind nursed her anyway and everththing was fine. Remember that doctors create rules based on sometimes extreme rare cases, with a medical intervention mind-set (take birth in this country for example!)
If it were me, I would nurse him anyway, if you find that it is unavoidable without traumatizing him. One thing to keep in mind when trying to avoid nursing him, is he is going to be much more willing to except not nursing from someone else. If he is with you/sees you, you know he will expect to nurse-Out of sight, out of mind. On the other hand, I would research to see if the scan is even totally necessary. Good luck!