Kate, first up, if you have already kicked the poor guy into touch on account of the post above then please go get him back.
In my experience, everything under the sun is related to T1 diabetes despite the fact that a million other things could be responsible. There could be a whole myriad of reasons for your fiancés early departures and the last thing I would do is stress over it too much cause that will only make it worse. If it's a huge concern go to the doc and see if they can help.
As for the above post, if you were my daughter then I'd say talk to him, try and figure out if there's anything going on in his head or anything else physically that's out of the ordinary. As he's your fiancée then I assume you love the guy and plan on spending your life with him. Life will throw a lot more worse things at you than a bit of premature ejaculation or quick cumming so writing him off as a poorly controlled diabetic and a liability to a young woman in love would be ridiculous.
Final thoughts - very few men (although they won't admit it) last very long in the pleasure zone - look at nature, animals are in an out with no time to shake it all about as it's a functional process - and very few women want men to be hammering away in the pleasure zone for too long. However, there's a whole lot more to a healthy sexual relationship than the in and out so put more emphasis on that. Also, and please excuse the vulgarity of this next comment, if he does go off quick, give him 5mins and then, if he can get it up again he may last all night.
