Hello- replied above about the different reins etc we have used but reading your follow up post realised I entirely forgot to mention how we deal (rather than just what we use)
My eldest son (our "wandering-bolter") is 4 almost 5 (call him ds5 on here as he'll be 5 when we travel) and he is dyspraxic, very, very bright boy however due to the dyspraxia he finds registering instruction and recall quite challenging, plus he's easily distracted, very bouncy (we affectionately refer to him as a "tweaker" because he's constantly on the move- if he's sat down his feet have to twitch, he flaps when he runs, if he stands still he's dancing on the spot) he's alot of work compared to the other 3 (and compared to all other boys his age we've met/socialised with and in the extended family). With him we used to back pack when we was age 15months to around 2.5ish (and then in exceptionally crowded places) - however reading your story you didn't have the luxury of knowing your son during that stage of his life and are having to teach these things later on.
We're a hippy household- not other phrase for it, we're non-physical discipliners (which often leads some people to assume, non-physical discipline= no discipline, which i assure you isn't the case) and so with his elder sister we'd always been able to talk, reason and see results (she's now 10 and is fabulous) but ds5 was more of a challenge- you can talk to him, he does understand it, he does take it in, and then his brain locks it away in a dark room which he occasionally stumbles into I guess (common issue with dyspraxia). He's nearly 5 as i say and now he knows he holds hands (even though his sister who is a year younger doesn't have to unless in a crowded place) For us it's taken alot of patience, alot of willing to reiterate oneself, alot of willingness to understand how he sees the world- it's very different for a dyspraxic child (alot of people hear dyspraxia and automatically think it's just co-ordination issues, it's not it's an entire multitude of issues all making up the condition- emotional, neurological, physical). He reacts differently to noise, social situations, physical challenges all alot different to us. He has very little reasoning of danger- he understands what is bad and will hurt but doesn't actually make the association it still might be a danger to him. It's made parenting him so very, very different to the other 3. Have you tried posting over at the disABILITIES board? I've been taking a read over there and alot of info.
With the wrist rein for my older girls when they were small we told our oldest she needed to take care of grandad or he would wander- she was a bright child and thought he genuinely might if she didn't keep a hold of him LOL With the backpack our son thought he just had his own bag- it worked well.
I would say you sound like you are doing a tremendous job adapting to his personal needs already so I really wouldn't change your methods just for one vacation, you can deal with the issues as a wider picture, before, during and around the vacation- i really don't think you or he needs the pressure of the deadline of the vacation to be out of old habits for. It will come in time, DS5 took alot more effort to get to take the same instruction his siblings did from the first time they were told but he is getting there.
I did worry when we first used the backpack about people staring and then I thought about it- they'd stare alot more if i were the lady on the news who lost her little boy, they'd stare alot more if i were the lady on the news whos little boy ran into traffic. Not all kids need them but each child is so very different, especially with children with special needs, that you just have to do what suits you as a family and ignore the people who stare because lets face it- you'll probably never see them again in your life.
It sounds like your family and your son are very blessed to have found each other, and I wish you all the best. 
