When he cannot make himself understood with others and when people take the mick, is it really that they cannot understand, or are they doing it for effect because they don't like him, or are taking the piss? Men do take the piss out of each other far more than women. If you didn't jump in and 'translate', what would happen? Would he be able to make himself understood?
A lot of people are very imprecise with language because they're just not interested, weren't taught thoroughly at school and don't give mental space to learning and remembering words and grammar. All those millions of people who confuse bought and brought, specific and pacific, etc and ect, stick apostrophes in plurals, write chester draws and so on. All perfectly easy to understand though.
Ease of recall diminishes with age. From about the mid-30s onwards I think. Even in my 40s I notice it. The more articulate a person is, the easier it is for them to 'talk around' the word they can't find, fluently. Someone with a smaller vocabulary, less facility with language, or who is just more tired or distracted, will stumble or say the wrong word.
Your husband's grammatical slips seem different though, it isn't vocabulary that he's losing. Is it possible that he is saying the words in full in his head and doesn't notice that he doesn't pronounce it all out loud? In your cat and football examples, he was effectively talking to himself, speaking his inner monologue out loud, so maybe he wasn't monitoring his own speech or trying to make himself understood. 'I eat too much this morning' could be the same. He's musing on what happened earlier by imagining himself back in that situation, rather than giving you a journalistic report of what happened. Maybe he's just quite detached from you (and others), a bit lost in his inner world and talking to himself most of the time?
Fun as it is to speculate, you're never going to know unless he sees a doctor. Only he can decide to do that.
You need to adopt tactics to help yourself, I think. Make a distinction between active conversation, when you're really talking with each other and passive narrative. Model good speech for your DCs, talk with them and read with them, rather than correcting him. Children learn by copying, not by being told. Far better for them to learn to try to comprehend people who don't speak perfect English than learn to nitpick and mock. When they learn languages at school, they'll be the ones struggling with basic skills. Taking for granted that others will listen with a desire to understand, will give them the confidence to speak and improve.
I do understand how little things can really grate. I'm sure we all do. Surely they grate most when there isn't much good stuff going on in the relationship and least when everything else is strong?
You sound quite fixated on this issue, so that it's no longer a small thing for you. You need to decide whether that's because it's become symbolic of a dead relationship, or whether it's just the dripping tap you can't turn off but could learn to ignore.