When we were at the BWI we walked to MGM. Myself, DH, DD 4yo at the time, my sis, 10 at the time, and my mom. We beat the boat. We all have very little patience and didn't want to wait in line for the boat on the way back and we beat it again. BWV are the closest to the walkway to MGM and it seemed like a fast walk, maybe 15min.
DD was in a stroller. I cannot stress enough how helpful it is to have a stroller, your own. Your DS will probably be more uncomfortable in a rental stroller, have you seen those things?
Epcot strollers:
http://wdwig.com/strepc1.jpg
MGM:
http://wdwig.com/mgmst1.jpg
MK:
http://wdwig.com/mk_stro1.jpg
AK:
http://wdwig.com/akstrol.jpg
He will definately get tired walking. It will be easier around the resort if you want to take a stroll around the lake, hang by the BW at night.
An umbrella stroller or any stroller can be wheeled up to the gate and I think they or you can store it on the plane so it can be waiting for you when you land. I bet a stroller would come in hand while at the airport. I can see yuour DH point about the carseat being a pain to lug around, but a stroller would help you more than a hinderence. JMHO!! I know you didn't ask, but I thought I'd give my 2 cents
dimimi on the family board there was a discussion about how old was too old for a stroller and I think I remember seeing kids as old as 7 or 8 still needing/using a rented stroller.
FYI
This is the stroller I used:
http://www.babysupermall.com/main/products/kol/kol36506.html
It says to 35lbs., but DD is 5yo and is 37lbs. and used it.