I'm a type one diabetic. The whole thing is pretty crap alot of the time: especially the people telling you what to do, criticising you when your blood sugar is not perfect and (it feels like) just generally getting at you all the time. (Everytime my DH asks "how's your sugar?" I have to swallow a desire to respond "none of your business leave me alone"; which I know is a completely unreasonable response to a genuinely concerned query on his behalf).
Now, I am not saying that you are getting at him or criticising him... but he may well feel like you are. Plus as Sidge commented, he may have only just got quite how silly he has been and doesn't want you to know as well.
Also, it sounds a little like he is having difficulty coming terms with the fact that his life has changed for ever. In fact I would suggest it sounds like he has been in denial about it (ignoring the problem by not taking the insulin or following the diet sheet rather that trying to work through it). Hopefully at the clinic they will be able to help him with this.
Five years of terrible control is unlikely to have caused long lasting and irreversible damage to him... If he gets his sugar under control in the next couple of months, that is. In fact, any related problems may well clear up by themselves (mine did). Of course, if he doesn't take responsability for his own health he is on the road to an early death (although I suspect the GP was trying to freak him out and get him to face up to reality rather than talking about the very near future)
In terms of the medicine: high blod sugar makes your blod thicker than normal whiih puts your blood vessels (especially the tiny ones in your eyes and kidneys) under a lot of pressure. They can burst leading to problems with eye sight and with kidney funtion. Equally high blood sugar can hhave negative consequences for nerve functions. One symptom for example is numbness , especially in the feet which can lead you to hurt them without realsiing it. A problem made worse by the fact that thick blood is sluggish in its movements and so, wounds don't heal very quickly. Additionally high blood sugar leaves you more prone to infections in wounds and generally to bugs and illnesses. All these things expalin why diabetic patients with longterm poor control can end up with amputations (but I emphasise long term here, 10-15 years).
In some cases it can cause nerve pain as well.(in my case my feet hurt constantly for a year).
Best wishes with it all.