Change in behaviour that can be described as a loss of inhibition can absolutely indicate neurological issues. How old is he? How is his health generally? Is he a heavy drinker? Diabetic?
Not just dementia but also Tia, metabolic disorders...
There was a thread a few months ago, op was originally just annoyed and fed up dh was expecting her to basically be his diary as his short term memory had worsened, iirc it turned out he'd had a tia. Something that hasn't occurred to either op or dh, he was quite young I think, but there turned out to be an underlying undiagnosed issue that made him susceptible to them.
Even something as daft as a hard bump to the wrong part of the head can cause issues.
My grandfather had a bad stroke at what I now know to have been a relatively young age (late 50's - when it's yours granda you just think they're old as they're a good bit older than you). Prior to my birth he'd been a pow and then a heavy drinker - malnutrition and the ravages of alcohol were both thought to be the main factors. He went from being very much stiff upper lip 'hard man' to sobbing with heartbreak at Andrex ads.
I've worked in geriatric care which is where you're more likely to see this type of thing, relatives are quite disturbed sometimes at an about change in their loved ones personality.
Must be said mental illness can also be a factor. Hypersexuality can be factor in several conditions.
But physical causes need to be ruled out first as they may indicate life threatening conditions that need treated Asap.
Also consider any medication he's just started.