I was with my horrid ex for 10 years, and I've been with my current SO for about a year.
I don't think he ever noticed, but I got the first syllable of my ex's name out twice, about to call him that - I somehow managed to mangle it into something else mid-utterance, but it was there.
I, personally, was horrified, as my ex and my SO are *nothing* alike. My SO wasn't reminding me of my ex when this happened, and I wasn't thinking of my ex when it happened, at all. I stressed about it when it happened, but then I realized that it happened because I had spent 10 years developing automatic vocal patterns that included ex's name. (I think the first time, he was driving and a car swerved near us, and I was trying to utter ex's name in a 'watch out' expression.)
When you've spent so much time automatically prefacing a zillion of your thoughts with certain sounds (i.e., your ex's name), it's quite hard to eradicate them. It may have absolutely nothing to do with how much you love(d) the person whose name you were using, or even whether you are or have been thinking of them as a real person, whether you secretly still want to be with them, or anything else.
When my grandmother needs to say something urgently, like 'that plate is hot!' or 'catch the falling glass!', I get called my aunt's name. Not because she's thinking of my aunt, or because I'm doing anything that reminds her of my aunt, but because she spent more than 18 years living daily with Aunt'sname, and saying things like 'Auntname grab the dog!!'.
It's a pattern that builds up in the brain, based on habit. The human brain puts zillions of things into 'autopilot' - otherwise we wouldn't be able to function. We don't analyze and rechoose each action every second - we allow those created patterns to allow us to put on our shoes while planning our grocery list.
So, in all likelihood, that's all it is. A simple pattern built up in the brain that has absolutely nothing to do with how he feels about you, or her, or whether he was or is thinking about her. I know it's uncomfortable and awkward, but it really is just a quirk of how our brains build and retain patterns.
When that happened to me, I made a conscious effort to start using SO's name, a lot - both out loud and in my own head. I'm trying to build a *new* pattern that my brain will revert to in times of 'autoresponses'.