I'm going to close this thread at this point because the OP's questions have been answered and it does not seem to be headed in a good direction.
I've been thinking of this thread while I was out today and have some food for thought - really based on the things below I pulled out from the original post.
If the OP would like the thread re-opened or would like to discuss more, please contact me.
......
group of 5 10-12yo kids! 
I'm chaperoning DS12's end-of-year field trip. Here's my dilemma: DS has Asperger's/Anxiety/ADHD/BiPolar(
just got that diagnosis) & sensory issues. When we do WDW as a family we use a
DAS for him, and
he does very well as long as we avoid major crowds, use ear plugs, etc......
I looked at UnderCover Tourist's Crowd calculator for this Thursday, 5/7/15 and it lists Universal IOA as 5 out of 10, so not particularly crowded.
But, having been to IOA, there are certain areas, like the Hogwarts area, that are very busy. Because it is designed like the original movie sets, the whole area is tight and can feel very crowded, even on days when the waits are fairly short.
So, although, it's not going to be the most crowded, you will still encounter crowds and many of them will be outside of line areas. So, you still will need to have a plan for the situations that might occur outside of lines/attractions.
If he normally wears ear plugs in the Disney parks, he will need them at Universal. I am certain that the others in his group will notice them and (being 10-12 yr old kids) they probably will ask some questions about them - they are not exactly usual theme park attire. So, you/your son will need to be aware that might come up.
DS is very high-functioning, and
the majority of his friends & classmates are unaware he has any disabilities and he wants to keep it that way. He functions very well in normal day-to-day situations as long as we keep to routines, use his accommodations in school, keep up w/medication, etc.
Things like theme parks really bring his issues out however. He was very reluctant to go on this trip because he's afraid having issues and/or meltdowns w/his friends around (thank you fragile 12yo ego & self-esteem

).
If the
majority of his friends and classmates are unaware he has any disabilities, that sounds like he has some who
are aware.
Are those kids coming on the trip? If so, I'd recommend you try to get those kids in your group.
Since they are already aware, there would be no need for you to pretend you have issues - they already know your son does.
If things like theme parks bring out his issues and he is already afraid of having issues or meltdowns, trying to keep a secret seems to me like throwing gasoline on the fire of his anxiety and might actually make him more likely to have issues.
I've got a niece and nephew with Aspergers and I can tell you for sure that most of their classmates knew there was something unusual about them. They are both extremely intelligent, but make comments and do things their parents kind of stopped noticing that were very evident to others.
For example, we were out with my sister and her son in a Mexican restaurant that had pictures of the food. When his order arrived, my nephew stared at his food for several minutes, then started cutting it into tiny pieces, about 1/4 inch square. He finished up with stirring around the food fragments for a few minutes before he ate a bite. My sister didn't bat an eye and acted like it was a normal occrance (which may have very well been true in her household), but everyone else at the table could not help noticing and there was a kind of uncomfortable silence about it. I told him I was curious why he cut his food that way and he said when it came, it "did not resemble the photograph adequately" (more or less his actual words), so in order to eat it, he needed to make it not resemble the photograph at all. After he cut it all up and stirred it around, he ate it all and commented that it was "remarkably tasty despite not having been prepared and presented exactly in the manner of the photograph" (again, pretty much his words).
There may be things like that which are not on your radar, but the other kids will notice because they are not usual things.
So my 2nd issues is, if US does offer their version of a DAS, is there a way to obtain & use it that won't cue his classmates in? .......
(we live near Orlando, so every year has a theme park trip). I'm really hoping for some advice!
Again, if you can get kids in your group that are already aware of his issues, it would greatly simplify things (even if that means you have a smaller than average group).
You
may be able to get and use Universal's version of the DAS and lead the kids to believe it is yours, not his. if any of the kids are frequent visitors to Universal, they will know you are using the pass because the experience will be different.
Also if you are supposedly the one using it, it means you will need to go on each attraction, even if he wants to go on the big roller coaster and you don't.
I'd rely more on planning and the Universal app do you can see which attractions you can go on without a long wait and without using the card.
The final thing I thought of is you need to plan for how your son will clue you in if he is having issues or is heading for a meltdown. If he has enough warning, he might be able to discretely let you know, but then what?
Will you be able to get another parent chaperone or teacher to take your group so you can take care of your son's need?