Extreme senses/sensitivity to stimuli

So, divorce from my ex with untreated ADHD is almost final, after leaving with kids a year ago. Working on co-parenting, which usually consists of me managing and taking care of everything and him contributing what he can when he can. Which is actually fine. We get along reasonably well and the kids are happy. 

I continue to unravel the past and figure things out--hindsight is always good when we learn from it, right? And this one aspect of his behavior keeps rearing its head. I would like to be able to understand it and explain it to my children. For now I just ignore it--it used to drive me crazy and embarrass me, but now I just find it curious. Has anyone else experience this? 

My ex spouse definitely has ADHD, and maybe a few other things going on--bipolar disorder, depression, OCD, asperger's? I don't know. But he responds extremely strongly to various stimuli. For example, bad smells may make most of us cover our mouths, but he starts gagging, tears roll down his cheeks, and he makes loud retching noises. Everything from a dirty diaper to bad garbage made him do this (convenient, huh?) He once swallowed some food the wrong way and ended up rolling around and thrashing in the floor, clutching his throat. A friend saw it and asked if we should call 911. "No, he just does that," I answered. It is just really melodramatic and beyond attention-seeking. It is like there is something mentally wrong, maybe ill about it. Several friends and family members have asked me if this is for real? Small injury to his knee? Limping around with a cane and getting a handicapped tag for his car. He also looks up the worst thing that could possibly happen, medically, for everything and sometimes scares our kids with it. (Surgery for a pinched nerve? Could result in paralysis and he will never walk again). 

It is worst when he is in pain or sick. A cold leaves him making the most crazy retching and coughing noises, again with tears running down his cheeks. It is a bit annoying when I am trying to pay attention to our kids or he is canceling out of an overnight with them at the last minute. 

There has to be some explanation for this. Honestly, I don't think it's for attention. I think there is something that makes him over-react like crazy to things like this. And forget talking about it. I tried a few times and he freaked out and said I was accusing him of lying. Worst fights ever. Once I rolled my eyes at him (which is not very nice, I admit, but I had just had a Cesarean and was up all night and breastfeeding and couldn't take it any more). Well, I was the meanest human being who has ever lived. He absolutely freaks out if someone calls his credibility into question. 

So, I just let it go, and try to ignore it now, but it happened again this morning and I know the kids are going to start asking about it at some point. Any ideas?
Thanks, everyone.