As a nurse, any mediction either oral or intramuscular that is partially given, either because people spit it out or because it leaks during the procedure, isn't given again because you cannot second guess how much was administered. I assume in this case that the nurse was absolutely certain that no medication entered into your sons muscle ( she will know this by the depth of the penetration and if she pushed the plunger.) It needs to go in so far to get into the muscle. Therefore she probably topped up the syringe to the amount prescribed knowing none had been administered & the needle wasn't in and it did spill. But I would ask her if I were you, and she will explain, if your not happy with her answer, speak to the GP. Kids getting injections is always an anxiety provoking situation for the parent (I had to send my mam with my first) because we feel responsible for consenting to something we are not sure is going to help or hinder, especially the MMR with all it's publicity. I'm sure he'll be Ok.