Gut issues are a common comorbidity with autism, it could just be he had some kind of gluten sensitivity and treating it allowed more energy to compensate for autism.
The other really big comorbidity one is ADHD - I’ve seen claims that the rate is high as 80%, maybe even 100% if you allow for very mild cases that probably wouldn’t be independently diagnosable.
When “comorbidity” is used in this context, are we actually referring to people dying because of it? Or does it just mean “are found together when discussing this disease?”
In common use ("don't be morbid", "that's morbid") I agree that 'morbid' has a connotation of death, so I don't think your understanding is wrong. In this case, a 'co-morbidity' is a term of art in the medical literature with the specific meaning of "occurring with".
The Latin morbus is disease and mortis is [of] death but my experience - and perhaps yours too - is that when discussions of morbidity (i.e. using the word morbid/morbidity) arise, they are often in the context where someone is near death or has died so it is a natural linguistic connection to make.
Remarkably often I find my partner has a different understanding of English words from me - she was schooled in French and can communicate in Italian and Spanish, but is also fluent in English (though spoken with French mannerisms which seem odd to me as a native English speaker). We often find we descend into discussions of the most correct meaning of a word in English which has a Latin root. The dictionary agrees with her mostly but I have an occasional win.