Hi Anna and Oliver,
Welcome to FDMB
Re the inappropriate peeing and pooing I agree with Dyana that it might be good to get a vet check-up, to try to establish whether there's any underlying health issues.
Does he
only poop outside of the litter tray? Just wonder if he's avoiding it for some reason. Has he had any constipation (and maybe had some pain/discomfort that he associates with his litter tray)? Has the tray been moved recently? Could he have been frightened while using it?
Any trouble with his back legs? Can he get into the tray OK?
And regarding peeing, diabetic cats sometimes just can't make it to the litter tray in time. And there may be a UTI (urinary tract infection) that's exacerbating things.
It sounds like you're willing to give hometesting a try. Good for you!

Most of us use glucose meters made for humans (they're
way cheaper to use than an animal-specific meter).
Here's a link to a page of pics and basic info on hometesting so you can get some idea of what's involved:
http://www.sugarpet.net/bloodtst.html
Hometesting should not hurt the kitty at all (there's very few nerve endings in the outer edge of the ear). And it's not hard to learn, but, like anything new, it may well take you a few goes to get the hang of it. We are here to help in any way we can.
Best wishes,
Eliz