There are stairs that lead up to the stage. I am not sure how many, maybe 5-6 steps.
I completely understand your predicament. You want to give him the best shot of being picked by preparing him but you don't want to have him all excited and prepared and not get picked.
My son wears a prosthetic leg, so I realize that this is not the same situation but I thought that I would share it in case it can help you in any way.
We went the week of June 22-29th. I told my son that sometimes you get picked and sometimes you don't. I knew that he would be disappointed if he didn't get picked. (The guy sitting next to us was waiting for his third time in an effort to get his son in the show.) My son was not wearing anything special, I don't think that any of the kids were. (It was really hot though!) They give them Jedi Robes to wear for the show.
We arrived early, maybe 20 minutes and spread out a towel for my son, my daughter and I to sit on. (When facing the stage, we were more to the right.)
Right before it started, I told him that as soon as they ask for volunteers, you need to stand up, raise your hand, smile and look excited. ( I think that the performers avoid the kids who look like they are unsure if they want to participate.) He did what I said and was the 2nd kid picked. They picked a lot of kids, maybe 12?? Every kid that they picked was raising their hand high and looking interested. There were several kids who were shy about getting those hands up. By the time that they decided to raise their hands and look interested, the kids had all been picked. It goes pretty fast.
Since your son is in a wheelchair, this is what I think that I would do. I would get there early and get a good spot right in front. I would tell the cm that are standing nearby that you can assist him in getting on stage. I just looked at my pictures and I can't see a ramp. There may be one there but I was taking pictures of my son, so I wasn't noticing what was around too much. If I were you, I would stand behind him and or beside him, kind of create a human barrier behind him, so that he is the only kid in front of you. (Maybe he will stand out more and it will be really obvious that he wants to be in the show.)
The training part is fun but I tell you that ALL of the kids were a little nervous looking when Darth Vader came out. He was just very intimidating. (My son said afterwards that his legs were shaking!) He is completely in character and NOT friendly. The kid that they had go first was probably one of the oldest kids.
If you think that he might get scared, you might want to watch one show and then try to get picked to be in another. I don't know if they will let anyone else on stage. The kids all have a little circle or something that they stand on so that they don't wack each other with the light sabers!
Another option in the POC show in MK. THAT was a blast! They do that one on the street level so no stairs. The characters are funny in that one, not scary. You still get to have a sword fight too!
I definitely don't think that it hurts to have your son approach a CM who is working the stage area and say, "I want to try to get picked to be in the show, where should I sit and what should I do?"
Good luck to your family! I hope that you have a wonderful time.