Good evening - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that Somba has become diabetic - I know that it can be a stressful disease to deal with. He's lucky to have you to care for him!
Based on what's most common, I'd consider cataracts a potential issue here. Diabetic pets quite commonly develop cataracts as a consequence of their high blood sugar. The longer their blood sugar is high before being diagnosed and the longer we take to get it under control, the faster the cataracts develop. I've seen some dogs develop them in a matter of weeks - from clear eyes to significant vision loss. I've seen other dogs develop them more slowly - like over months to years, but just about every diabetic dog is going to develop cataracts from their diabetes. Cataracts look like gray, blue, or white color change inside the pupil that is normally black. If you shine a light on their eyes, you sometimes can see a crystalline white change inside their pupils. Cataracts can cause significant inflammation in the eye and secondary glaucoma, which can be painful and require prescription medication to treat.
If he's acting relatively normally and you're seeing mild changes, you could ask your vet about it at his next exam. However, if you're seeing major changes, discharge, severe redness, or have any doubts about his vision, I'd suggest an exam sooner. If he's not acting normally, it'd be a good idea to get a blood sugar run on him ASAP to be sure that he's not hypoglycemic (low sugar). If they have low blood sugar they can stare off into space, become severely lethargic, have muscle tremors, weakness, or even seizures. I have my clients educated on what to look for and also suggest that if they think that their pet is having a hypoglycemic episode to put some karo or corn syrup (or maple syrup in a pinch) on their pet's gums to bring that sugar up.
Please let me know what other questions I can answer for you :)