I understand what you're saying about
taking something natural but you know that even natural herbs or other plants can have side effects, some of which we might not be aware of. I am a big proponent of natural food.
If his blood sugar levels are high enough to be on 2500 mg metformin, he's either very insulin resistant or not making enough insulin (I mean his pancreas is not making enough insulin.) or both. I don't know of any natural herb or plant that has that kind of control. I did try berberine, Ceylon cinnamon and gymnena sylvestre at different times over the past 14 years but didn't find any effects on my blood sugar.
Here is a website that talks about
these supplements. The bottom line is what his blood sugar readings are and that they are fairly consistent and don't fluctuate widely. https://www.healthline.com/nutrition/blood-sugar-supplements