I've only flown southwest and delta, so can't speak for all airlines, but just go to the counter when you get to your gate and let them know your son has a medical issue and requires extra time to board. They may let your whole family preboard or just let one person accompany him. Be prepared for southwest to allow all passengers with young children needing extra time to board at the same time as medical (I have experienced this twice, and it basically defeated the point of preboarding since there are so many small kids traveling to WDW). They'll make an announcement when its time for preboarding, something along the lines of "we're going to begin boarding for those passengers requiring extra time". You could be on the plane 10 minutes before general boarding, or 2 if they're running behind.
with Southwest their policy us to allow person with need to pre board along with one other person to provide assistance additional members of family áre asked to board as noted on the boarding passes.
We were also able to board with 3 of us (DH, DD and myself) on Southwest (and actually on every airline we have ever flown).Not my experience.
We flew with Southwest in August and took 3 different flights. We had no issues at all with all 3 of us (myself, DH and DS) pre-boarding together. I went to the gate and spoke to the agent there. Twice I was directed to go and talk to the agents at a Southwest customer service desk to get a pre-boarding pass and once I got the pass at the actual gate. The one time I just had to go to the gate was at a smaller airport. The two larger ones had the central desk. At no point were we told that only one of us could accompany our son and that the other would have to wait.
Every time I went and spoke to them, I politely explained that my son would benefit from being able to pre-board and told them why and I did not experience any negativity at all. Quite an easy process.
I think it probably depends on the size of the group and how many groups they are expecting to preboard.Just flew SW this trip. Flight down all 5 preboarded. First flight back they allowed my oldest with her dad and said the other 3 could go with family boarding. We were in section A which was sooner, so we just boarded then. The second leg if the flight home they let all 5 go.