Hi Poppy - my dd was diagnosed about 18 months ago and I think I'm still grieving now. A diabetic can do everything anyone else can and have a great life, but they have lost the normality of not working out how every mouthful of food, activity, and emotion affects them. It is normal to be sad and I think it possibly is worse for the parent than the child. I know my DD has a lifetime to live with this and she's the one who will have to deal with it in her adult life, not me, but I also know if it were me, not her, I would accept things much more than I can knowing it's my precious child who is affected.
I would say no one understands like the parent of a diabetic, I don't think it's the same to be diabatic as to be a diabetic's parent. May I point you in the direction of CWD - it's for parents of children with diabetes. Obviously most people there have children, not adult children, with diabetes, but there are some on there who have children who have left home and their email lists are a mine of information and support. Link (I hope) is:
www.childrenwithdiabetesuk.org
Also make sure he is under the best care possible, access to things like pumps varies massively region to region. The best places for children I understand are UCLH, Leeds and Addenbrookes, I'm not sure if it's the same for adults, but if he isn't getting access to treatment he wants he can go out of area. And I agree make sure his close friends/house mates know what to do in event of a hypo. Hopefully he has good friends who can keep an eye out when drinking etc.
Good luck, hopefully it will make him stronger and if you ever want to offload here I always try and keep an eye out for diabetes threads, though there don't seem to be many.