The fact that he is willing to get it investigated is a great thing...

Obviously he has sperm and is fertile so whatever the outcome, it doesn't mean you can't conceive.

I think a trip to the GP is in order to get the ball rolling. If he wants to go alone, get him to tell the Dr that he's done some investigating and he thinks it could be either Anejaculation or Retrograde Ejaculation due to his diabetes. Make sure his Dr doesn't fob him off with 'oh, you're young, you'll be fine'.

Coming from someone who has trouble conceiving with a partner who has no trouble, I can't stress enough that you are there for him to vent to. Whatever's on his mind, he will need to get out... don't just say things like 'oh it'll be ok' or dismiss what he's saying (I'm not saying you would). Let him get it out...

Stay strong xo