get the macleren - we had it for our 3 yr old in Sept and she LOVED it. We did get the extra canpoy and also the holder that attaches to the arms - it holds 2 cans, bottled water, sippy cup etc... It also has a mesh bag to hold cameras, wipes, sunscreen etc...
To be honest with you, I prefer a stroller even if dd only stays in it for a little while. It holds everything you need to keep things off your back etc... Our little bag with any valuable fit right below in the mesh under the seat. We would park the stroller on rides and take the bag with us. The stroller also "holds" any balloons and also a place to store souveniers. I just couldn't imagine not having it for my almost 4 yr old this trip. You NEED a stroller - believe me - your boy may walk and walk and walk - but he WILL sit in that stroller everyday at some point. You don't want a meltdown or a sleeping boy being carried - they are heavy!! Plus if it is really crowded, you actually SAVE time pushing him then wandering around with him.
Hope you can sway him - if not, just get rentals when he tires out and look at your husband and said, I told you so!!!!