OP's DH isn't talking "properly". He's failing to use the right register for the context.
He's not necessarily using "bigger" words. He's using words that are only appropriate for formal written language and that are archaic in spoken language. It doesn't matter how big the word is. It's just inappropriate
This ^^ thanks Gwen
I have a wide vocabulary too, i just feel no need to crowbar needlessly elaborate words into my speech or written work. I see far greater merit in using simple, beautiful words with care. If there's a need to use a complex or obscure term because a plain English equivalent simply doesn't exist, I'll use it, but what possible reason can there be to use antediluvian (unless you're talking specifically about the period before the great flood) apart from to demonstrate you know the word antediluvian when out dated, old fashioned etc. would work far better.
If I were teaching an undergraduate seminar and I wanted to make a point about re-offending rates:
'Smith claims that attitudes to therapy in prison are hopelessly antediluvian and as a result the rate of recidivism remains high.'
The focus of a good proportion of the seminar group would shift to attempting to figure out what ' antediluvian' and 'recidivism' meant, rather than focusing on the point I was trying to put across: that Smith makes a link between outdated attitude to therapy in prison and re-offending on release.
I may well follow this up by introducing the term 'recidivism' as it is likely to crop up in other literature on the topic, but to use antediluvian in that context would just be fuelling my own ego to the detriment of the people I was speaking to.