I have not taken my daughter to any of the Disney parks while she was still a baby, but I agree with several PPs about bringing your car seat. I can completely understand that it would be a bit cumbersome and make your park touring just a little inefficient, but I have a feeling that it would be worse (and even more inconvenient) to
not have the car seat with you. Your youngest son would be more comfortable and able to easily take naps (plus go to sleep at night) while the rest of you can carry on with your day.
It would be even better if your car seat snaps into your stroller; I used the car seat/stroller combo all the time with my daughter when she was younger and it makes things so much easier! That way, you're not lugging the infant car seat around everywhere in addition to the ergo and stroller. For places (restaurants or otherwise) that won't allow you to bring the stroller inside, I would think that the transition would go much more smoothly to just carry your son right on inside while he's in his infant car seat. Plus, he will have a place to sit while you all enjoy your meals. The (possibly) awkward bus rides would really be worth it to be able to tour the parks as efficiently as you can, in my opinion.
Keep in mind, though, that you don't want to keep your son in the infant car seat
all day long. As I'm sure you already know, it's not good to keep a baby in their car seat for hours on end, as Gracie09 mentioned in her response. Yes, car seats
can get hot, but the weather in Orlando should be fairly mild this time of year. I'm sure you will use your good mommy common sense in that respect; your son will be just fine! You already said that you're bringing his stroller and ergo also. So between those two, the car seat, and your arms, I have no doubt that he will be very comfortable and you will all have a fantastic trip.
All that being said, it really does come down to personal preference. Some parents tour the parks with their baby in a car seat, others use a stroller/ergo/wrap, and many parents use a combination. It boils down to whatever
you and your baby are most comfortable doing. Either way, with or without the car seat, I don't think you will experience very many problems. If you feel that bringing the carrier is tolerable, I would personally recommend it. But, if you think it will be too much of a pain to tow a car seat around with you, then just use the ergo and stroller. You most certainly can get by without it!
I hope you and your family have a wonderfully magical vacation at Disney!
