My DS1 (oohh--almost 2, in another month!) w/ Down syndrome doesn't (so far) have any other health issues.
My main worry about our trip [which is still 10 months away] is the "carrying factor." His ligaments and joints are sooooo very looooose! It makes him very hard to hold on to. I was trying to hold on to him yesterday during
DS8's b'day party, and it was like trying to hold onto 20 lbs of wet spaghetti. After a couple of minutes I had to put him down. Not only is it very tiring and uncomfortable, but I worry about safety, because it is going to be nearly impossible to hold onto him for 30 minutes waiting in a line. And the thought of holding onto him on the bus--especially if we end up standing (I was so surprised last trip how grown, apparently healthy men can completely avoid making eye contact with a woman standing on a bus with a small child).
To make matters worse, my DM will probably be bringing DS back to the hotel midday for naps, and she has a worse time holding onto him than I do. Even once he starts walking, he's so tiny that it makes it really hard to stoop down to hold onto his hand, and I worry about dislocating his arm if he tries to bolt [oh, that could never happen you say?

Only to me--I disclocated DD12s arm back when she was 3, at the State Fair of Texas. I was holding on to her hand and she tried to bolt, and I didn't let go. Pulled her elbow right out of the socket. The State Fair staff had us transported by ambulance to the hospital, because she was screaming and it would have taken us a half hour to get back to our car, and we spent the next few hours in the Baylor Hospital emergency room. Ruined the day, she was in soo much pain until about 20 minutes after they popped it back in, and I felt like the 1st prize winner for Bad Mommy of the Year!
I would feel so much better w/ the wheelchair stroller so that we could fasten him down on the bus, but because he doesn't need it in our day-to-day lives, I really can't justify putting out the money for it. A wheelchair company in Orlando gave me the # of a company that may rent the wheelchair strollers, which would be great!