Hope its better than expected today essbee. Do they sedate him for the ECG (sorry I know nothing about these things- is that done routinely/can you request it?). When ds1 needed his x-ray it took 4 adults to hold him down (one had to run away before the photo- another nurse went well beyond the call of duty and donned a lead vest to help restrain him). He only needed to be still for a few seconds and we still didn't manage to get the shot they needed- have no idea how 15 minutes would even be possible, and your ds is bigger than my ds1.
They do know not to make him queue don't they? (That made a huge difference to our last hospital visit- would have been impossible with queueing).