Hmmm...my 6 yr old is on an insulin pump so our experience might be different but I would be uncomfortable leaving the meds and test kit, etc. in the wheelchair unless the chair is kept with you most of the time on the rides, etc. If it is parked outside the attractions (like most strollers) I would be hesitant.
It is possible to leave the insulin at the First Aid station but I realize you might want to keep it with you - it would be a hassle to have to go there every time you needed to do a correction ate something (especially if your son is on MDI and snack alot - takes a lot of shots). I can't offer advice on how well bottles or insulin pens would hold up if it gets really hot. Seems like you'd need to keep them cooled off a bit - some people like the Frio pack:
http://www.coolerconcept.com/ They sell lots of different ones.
We carry a small bag that we can take on all the rides with his test kit, glucagons, epi-pens (he has a nut allergy), tabs and juice, etc.
Because my son pumps the insulin is in that and we do have problems when we go in the summer keep that cool (cause he wears the pump next to his body). We've had times the insulin seemed to lose effectiveness so we need to change out the insulin in the pump. The heat really does a number on him too. I suggest going to Guest Services and telling them what his needs are and see what assistance they can provide. You can PM me about this if you want.
This is a great website for doing WDW with type 1:
http://www.allearsnet.com/pl/diabetes.htm
YOu should consider coming to the Friends for Life conference which is usually held in Orlando in July every year. Every other year they hold it at a WDW resort (CSR - this year it is not there but 2009 it is back to CSR). There is more about it at the website above.
