Oh how I feel your pain!
We just went through this with our DD3. Or should I say we are going through this with our DD3. What began as a simple sound issue morphed into a crazy fear of any toilet outside of a home environment. We tried to cover the electric flusher with a post-it-note - no dice. I can't tell you how aweful her hysterics were. I'm certain that while trying to get her to use the potty before a long flight out of Newark I had moms peering in to see if I was abusing her. It was aweful. It has subsided a little, but not enough to let me off the hook.
I will tell you the two things we have done, as absurd as they sound, in case you have your back against a wall. First, we bought her children's rifle earmuffs. You know those headsets hunters or shooters wear to protect their hearing from the shot? Well, who knew they made children's versions! We got some for like $6 at our local sporting goods store. She looks ridiculous, but it muffles loud noises enough that we can enjoy events without her freaking out, or walking around with her hands over her ears all day. We recently took her into NYC for a Yankee game, and she literally wore them from the moment we stepped onto the street until we got back in the car at the end of the day. (I never really realized how loud the city was. (Buses, horns, the subway, crazed baseball fans, etc.) If sound is really the issue, you might try that. (It also helps with fireworks, parades, airports, etc.) I can't tell you the number of weird looks we get from people when we're out in public. But most people get a good chuckle out of it - especially when we tell them she hates loud noises. It doesn't seem to bother her at all - and it frees her up to enjoy the world around her - albeit it at a lower decible level.
The other thing that we ended up doing was carrying some GoodNite pullups with us. I literally take her into the stall, take off her underwear, put on the pullup, she does her thing (never #2!), we take the pullup off and put her underwear back on. Then we simply toss the offending pullup in the garbage. I know it sounds ridiculous, but she just wouldn't budge. It has saved us a lot of grief - and has simplified our life for the moment. She just started preschool, and I was really worried about it - but she has gone two days and used the potty both days. With any luck she'll outgrow this completely soon.
Do hang in there. I know exactly how you feel - what a pain, right?! I mean, you get all excited because diapers and pullups are behind you, and then you have to take this huge step back.
I hope your DS will respond to the post-it-notes. But if you're in a jam you might try our crazy method - it just might work.
Good luck!
Make sure he dosen't have to go and show him how the toilets work - how the post it will work and ask him what he would like you to do. 
