On another note I have been looking into some of the different sling/carriers to use when we go on our next trip since dd is as big as she is now for standing in line and especially for the buses when there is only standing room so far the mei tai look pretty good and the ergo there are price but it helps take on some of the strain. I don't know that the sling type carrier would be helpful for you since you ds doesn't sit on his own but one of the ones that you could use in a forward facing position for him to be able to see as well might be helpful at times.
Have a great trip.
I'm glad you mentioned those, since you're coming from a place of experience with this condition (is that the right word?). I was thinking Ergo, too.
I'm only 5'3", but have been able to carry DS with the Ergo, fairly easily, and he's now over half my height and over 30 pounds. Now, he's not really interested in being worn anymore, so I haven't done it recently, but it's MUCH easier than carrying him in my arms.
The Ergo has the head thing, I don't know how to describe it, it can sort of support the head, so if your son isn't good at that still, that might help with head/neck problems. But the body of the Ergo is really supportive, and the buckles and such don't give it as "hippie" a look as others (nothing wrong with lookin' like a hippie, but options are nice!).
I personally never got into the forward-facing thing with front carriers, and I know you can NOT do that with the Ergo. So if DS wouldn't be OK with facing you OR a back-carry (where he could face the same way as you) then I guess it's out. My son never got used to facing forwards while on my front, so he developed a great rapport with people behind me. It was super-secret, who he was smiling at, and what the people behind me were doing, but he had great fun.
There are also very supportive mei tais available, though those involve tying knots and whatnot...I found the Ergo last year at DL to be alot easier than the mei tai the year before.
Anyway, that's just an offshoot idea I was thinking about, but wasn't going to post b/c I have no experience with the low-tone issue, until the previous poster mentioned it.
Good luck, and please don't worry about what others are thinking. I'm from a family where extended nursing is the norm, and while I try to not nurse my almost 3 year old in public anymore (IT doesn't embarrass me, but HE does sometimes), when I have, I just ignore everyone. I might miss some nice looks, but I know I miss the nasty looks, and that feels safe to me. Plus, I know from my own face that sometimes a person can be thinking the sweetest, most loving thoughts, but their face just is NOT registering it. The women in my family have that problem, looking like we hate you while we're thinking wonderful thoughts about you.

So perhaps, if you do get weird looks, you could just assume that they are like me (or are me?), and what they are thinking is not translating well onto their facial expression!
Have fun!