My Son is a type 1 diabetic, and I called in advance to ask about what we needed. They told me to bring along a doctor's note, explaining the need for the needles, meds, etc. They checked his bag upon entry and never asked for a note to explain, but I recommend that you bring one anyway. Our GP simply wrote a note on a prescription pad stating that he needed to have the meds with him at all times. Disney IS checking every bag upon entry.
We had no problems, and had a very enjoyable visit. In fact, the Rainforest Cafe seated us ahead of a 90 minute line when we miss- judged our meal schedule, to help us get back on track. The also expedited our food. Big thanks to them for helping us out.