My first thought was maybe he's doing on purpose to wind you up - my DP would do this as a joke
My second thought following a couple of your updates was that your continuous 'corrections' are making him so self conscious that he make nervous errors.
My third thought was 'Give the guy a break, seriously, give the guy a fucking break.'
Maybe going out on a limb here - but I think you're the problem here not your DH. The last example you gave was a perfectly reasonable thing to say. Our spontaneous speech isn't rehearsed - sometimes words are in the wrong order, tenses are mixed etc. In fact none of the examples you give are anything other than fine. Maybe not the most eloquent speech but perfectly fine.
Let's be honest - your written language isn't perfect "He's gotten worse", isn't to the same level as Shakespeare, so why should your DH's spoken language.
Leave the man be. Work on communication, not linguistic pedantry. You claim you're not a pedant, but from the examples you provide, you most certainly are. I think if you were to get your DH into a GP's surgery and explained 'the problem', the GP would using you as a case study of the crazy things people go to their GPs for. I would certainly love to be a fly on the wall for that consultation
Doctor How can I help you today Mr & Mrs Likeohmygod ?
You Well doctor, my husband is speaking without commas and
pauses, sometime he grunts when I speak to him or misuses
verb tenses. I'm constantly having to correct his speech, but
he never improves
Seriously, leave the man be. As for your children copying him - have you considered they maybe are using standard speech patterns and that you are the one out of kilter?