The best thing to do is to wait until he knows what level of diabetes he has. Then if possible, I don't know how close you live to him, go to his first few doctor appointments with him and encourage him - if the doctor hasn't - to see a nutritionist. That will be the best way to understand what he can and can't eat, what portions of what he can eat, etc.
If he has to go on insulin someone should learn how to give him the shots but mostly he will do it himself. Someone else should know in case he is ever sick or incapcitated and can't administer them at some point.
But my highest encouragement is to go to a doctor visit with him if you can. That way you can get information as a family member straight from the doctor. Also if he can find a Diabetes support group, encourage him to go to them. My husband has learned so much from the groups he goes to and it helps his mental state as well. Diabetics can get depressed - so watch for those symptoms and make sure if they start happening he talks to his doctor as soon as he can.