I agree with Defren, the weight will stabilise. I had the same problem. Weight shot down, and I thought I'd end up like beanpole. It steadied on a BMI of 22. I ate loads of nuts to help it. Nuts shouldn't hurt BGs providing your hubby stays of cashews. Walnuts seem particularly good.
Metformin does help Fasting BGs so coming off them could give him the higher readings. Why does he want to come off them? Apart from potential tummy probs, their is no downside. Doesn't strain the Pancreas, very safe, and good protective against future cardio probs. I'd say stick with them unless he has other problems such as kidney issues.