You know what, Karen? There is no right and wrong way. As you've seen in this topic; some will bite your head of if your son asks a question like "why do you use a chair?", while others will take time to explain. You see, those of us using a chair are just like anybody else; humans all with their own caracters and moodswings. You know from every day life that asking 100 persons the same question, will not give you the same 100 reactions. A lot of the times your guts will tell you what to expect or do. And sometimes you'll make a mistake, well that's life. This can be a touchy subject -you don't want to do anything 'wrong' and the person using a wc can be oversensative or just in a real bad mood-, like a lot of things in life.
Compare it to other things in life, how do you educate your son about those? If he has a question like "mommy, why is that lady so big?" when he sees a woman in the grocerystore that is obese, would you tel him to ask her? Would you get all nervous because weight is a touchy subject for a lot of people, or would you try and teach him about different people and different bodies?
If you ask me, it is about 95% following your gut instinct and logic thinking. Do what feels right at that time in that spot. If he asks a question about a wc when a wc is right in front of him waiting for a ride, maybe it feels right to ask the person in front of you if it's allright if your son asks a question. (this'll give you some space for those who do mind questions)
Take it to his level without telling him lies. A wc is nothing more but a way of transportation. Some walk, some ride a bike, some use a stroller, some use a wc. For the young kids like your son -3 yo, right?- I take the time and talk to them. IF the happen to ask this specific question, I'll ask them about strollers. They'll be the first to tell me a stroller is because their kidsis/brother/whatever small child can't walk or walk long enough yet. This is a good opening for me to tell them my legs can't walk and I use the wc just like "point at sis/brother/kid" uses a stroller. For me, it is SO not important to inform a toddler about too much of the negative stuff. The most important are 3 things;
- people are all different, but all are equally worthy. It isn't our place to judge people -at the right age add educating the difference between judging an act and judging a person-
- be sensative to your surrounding at a level that is appropriate to the age and cognative level of that child
- be honest, don't lie
Because I've got some nice stuff on my chair, kids don't ask a lot about the 'touchy' subjects, or remember to do so after the 'cool stuff'.

They are way more into the lights in my wheels -mommy, we need those on Annies stroller-, Animal or Kermy on my joystick or wanting to have a little contest to see who can 'run' the fastest, them or my chair.
I've had some kids try out the chairs over the years. Why? Because it's a great way to teach them -and the are the best teachers when it comes to teaching adults!!!- and I can understand their natural enthousiasm that makes them kids. A wc can have positive attraction to them, and I'm not the one to say that's a wrong thing. Sure, it's a great moment to educate them, but there's nothing wrong with a basic positive look at a wc. Hack, I'm even still like a small kid in the toystore if I see the newest toys for the wc's. A kid sitting in my chair for even only 5 minutes learns more than 5 hours could teach him if you talk. The child not only can experience the fact that a wc is also positive -without it we ain't going anywhere afterall- but they also will notice the down sides that come with it and absorb that in a constructive way. Now don't come running to me with a busload full of kids.

Sometimes I'll feel like doing this, enough times I won't. It's just like 'your role', somehow the situation will tell me it might be a thing to do.
Kids this young will tend to pick it up very quickly in a positive way -allthough sometimes being scared of the unknown, but than again, they'll also be scared of the lady with the really weird big tatoo in her face-. Sure, they blur out things that can make you go "god help me" as a mom. But come on; their toddlers! To me it's more important how a parent reacts to such a comment than the comment in itself. Dragging a kid away in shame is something that I don't like. Most kids will remember it as "a disability is something to hide and not talk about". Just tell him/do the same thing you'ld normally do if he makes a comment that isn't appropriate.
Adults, those are the ones that need some sensitivity training. The poor things just are so shocked by a wc, a group of them just freeze to the ground or loose their ability to speak and think. And you know what? Being nice, open, moody, rude, nothing worked as great as having some simple but very

-if I say so myself- stuff on my chair. For instance those lights in the wheels of my manual chair gets loads of reactions of people that are so enthousiastic about some lights, they totally forget the annoying "ouch wc, person in it, must be retarded, must I talk to them to not hurt them or do I hurt them by talking to them, ah poor thing, such a poor life". I get a lot of stares, I can't go into public with that chair with not getting stares. But very very little people still stare at me in that chair because of the chair. Nop, the lights are way more thrilling.
My Animal/Kermy on the powerchair also has that effect but in a smaller way. I like the way it breakes the ice AND like giving my chair some additude/toys, so I'm looking allover the place to pimp that chair to the level my manual is at.
OK, I'm on my preaching way, can't stop talking. As you've seen here, there is also a lot of difference about the way 'we' rollers look at things. You mght get your head bitten of about a question DS has, I've had my head bitten of about letting anyone other than me use the chair or my black sence of humor that also applies to my chair. If you ever get your head bitten of, think about it for a moment. Did you play a roll in it that could be done better next time? If so, you learn. If not; let it go. It's life, sometimes it just sucks and you can't do right. That doesn't mean you were at 'fault' or should've done better in raising your child/interacting yourself.