The problem may be that he is locked into a treatment cycle. The GP may think he is following recommended procedure (as indeed he is) by putting your husband on gliclazide (Zicron) if his BGs had failed to respond to metformin after 12 weeks, and that he should continue on gliclazide until his levels improve. It seems very strange that he was diagnosed as prediabetic wiith fasting bloods of 4.8, and that he is on gliclazide while still prediabetic, but then it is also strange if he is not then getting hypos on gliclazide, and that the side effects still imply high BG levels.
Maybe you could try discussing things again with the GP. Begin with the fact that you understand why the GP was following recommended procedures in good faith, but that the initial decision to put him on metformin was odd (whatever one thinks of the subsequent treatment). Get a blood glucose monitor and explain that, given that gliclazide can cause weight gain, your husband wants to try a systematic weight loss programme first -- monitoring his BGs regularly -- to see whether that can reduce the insulin resistance. If that doesn't work then he will go back to gliclazide. Trying to get the GP involved in an agreed procedure that doesn't challenge his judgment might be the best way forward (don't cite the locum's assessment against his -- even if the locum was correct -- that will only make him more defensive). Good luck!