I would be grateful for advice!

Get him to drink plenty of water.

Obviously reduce his sugar/carb intake for the time being. Also phone his doctor and explain the readings and I'd imagine he'd suggest upping the Metformin.

I'd speak to your father's doctor and explain what's been happening to his blood sugar. Do you know why the Metformin was reduced? If there wasn't a problem with it, then the doctor might increase it again.

Is it possible the increased sugars are due to his unwellness? Things like infection can put blood sugar up.

Those levels that your father is reaching with his Blood/Glucose since having his Metformin dosage has been cut to a quarter of the previous dosage are as you rightly point out FAR TOO HIGH and something needs to be done.
As Slip suggests, changing his diet to one of a low carb diet will obviously help long term as will any form of exercise. You don't mention if you father is overweight or suffering with a temporary illness and any RISE in overall diabetic control for whatever reason needs to be correctly addressed.
Sadly many NHS general practitioners don't want to put themselves out and make a house visit, but you do need to get help with this. The first option is to call at your GP's surgery and demand a visit from either the Dr(s) themselves or a Diabetic nurse, explaining to them that his B/G levels are now consistently starting to rise since his medication was reduced.
Sometimes you have to make yourself a B-----dy nusance to get some action done, and frequently the mere threat of reporting the Dr or Diabetic Nurse to either the Practice manager or the Local NHS patient Trust is sufficient to get the appropriate action.
If all this still fails to get them to call at your father's home, then it's worth considering using the 111 Non-Emergency NHS helpline number and explain your worries to them. Depending on what you say they might well decide to send one of their roving doctors directly to your father's home or advise him visiting a Wellman clinic or your local hospital.
Your father may not appreciate your concerns for him, but believe me when I say that by following up on this you are showing us just how much you care for his short and long term welfare.
Please let us know how you get on.

Firstly I'd get his doctor to check that he doesn't have an infection from the catheter. I've been advised myself that they make you highly prone to infections (I have a bladder condition and use of catheters has been discussed). So that is the first thing to check because if he has an infection then his blood sugar will be high. If he doesn't have an infection, then his doctor needs to review his treatment plan for him and it may be he needs a different type of medication or even insulin. Even if he does have an infection, his doc should help him out with controlling his sugar levels and treating the infection. You may want to stress to your father that an infection is a serious matter and he doesn't want it to spread to his kidneys I'm sure.... maybe that will help encourage him to see his doc.

@Nigenali - When we are ill our body makes hormones to fight the illness. These same hormones raise our blood sugars, which is why we need to test our blood sugars more. There might be some infection if the catheter is painful? I think a visit to the Doctor is needed.

ps... the last time I used a catheter I had an infection that lasted a month and with a bacteria that apparently indicates kidney stones (they had to do a CT scan on me to check, thankfully no stones). I don't like catheters very much for that reason as it's not often I don't get an infection from them. I've had catheters inserted a lot for tests and procedures on my bladder. I think it may also be that diabetes makes us more prone to infections as well. And with your father saying it's painful that is not a good sign.

If the doctor won't visit then, if i was in your situation, I'd call the surgery when it's closed and make use of the out of hours service and have someone visit your father.

Try doing the above, one of my GPs had a mini rant at me for not using the out of hours service, telling me it costs himself thousands of pounds each month and people don't use it often enough.

Good luck.

Are the confusion and raised sugars a recent thing?

If it is, then I would tend to want to have his urine checked. Sadly, longer term catheterisation (and clearly I have no idea if he is long-term catheterised) can be tricky to manage, with increased chances of urinary infections. All that "alien" tubing never helps. Urinary infections can both raise the blood sugars, in some cases quite a lot, but also lead to significant dehydration, which in turn can lead to confusion or increased confusion. See where my thinking might be?

If any of that seems likely, could a wee sample be taken when his catheter bag is emptied or changed? It can go into any sterile bottle (boiling water in in for a few minutes then dried would be good enough) and be dropped off at his Doc surgery for an initial dip test to ease your mind, perhaps? Alternatively, if he is longer term catheterised, he may have some UriStix at home that you could dip into a sample yourselves to see what turns up. You'd be looking for protein, increased white cells or blood as markers for more urgent attention.

Clearly, I am not diagnosing anything. I'm not a Doctor, or even a practising clinician these days, but urinary conditions in the elderly are very common. They can cause extreme symptoms, but do need attention.

In the meantime, it would be good if your Father's main carer could help by keeping a close eye on his diet - just to reassure you there aren't any bigger clues in there.

Good luck with it all.

Excellent news @Nigenali

That's great news @Nigenali - I'm sure your father will start feeling much better soon.