You won't be able to use his carseat on any of the WDW buses (or boats, or monorails, etc) either, it's not just DME, it's all WDW transportation.
If the car seat is a deal breaker for you, you may want to rent a car. The youngest my DS was when we uses Disney transportation (NOT DME) was about 5, so I don't have any real world experience, but I know there are other posters who will have better experiences for you.