Sugar levels can contribute to mood. Irritability, fatigue, etc. Diabetics can lash out, be incredibly angry, introverted and so on. The key to all of this is understading the disease as well as you can and keeping control of your blood sugar levels.
The more normal your ranges are the easier it will be to control your emotions.
I am a spinal cord injury and just diagnosed with Type II Diabetes 11 months ago. I lately, about a year ago, started having pretty intense depression as I was making plans for dying, but not suicide. My PCP prescribed Effexor which made me feel squirrely at first but then started taking effect. My wife takes Lipitor (sp?) or otherwise we would end up shooting each other.
Check with your doctor. It is amazing how good it feels to not feel like there's no future in sight.
Good luck.
Its normal for people to feel overwhelmed when confronted with an illness. He needs to see his doctor and speak honestly about his mental well-being. He/she can recommend a support group to talk with others to let him know he's not alone. If necessary, he/she can Rx a drug to counter the depression. In the meantime he needs to continue trying to control and manage his diabetes, continue with daily exercise, and avoid alcohol which can worsen his state. Good luck