Short temper :(

Impossible for any of us here to give you a definitive answer as it could be due to many things.
Is he under stress through his job, with some other member of his close family, a friend or is it the result of rebelling against the diet he was once familiar with and accustomed to.
It might be best to simple take a temporary back seat and listen, watch and wait, and eventually what is bothering him will come out.
Some people. particularly us men, often have difficulty in expressing our inner feelings so you may well need to be patient over this issue.
All the best - Please let us know what transpires and how you get on.

Hi @ecmc1960 Welcome to the forum. I can certainly vouch for high blood glucose levels causing me to lose my temper. If he's running high then yes this would be a contributory factor, of course we are all different, but I tend to think that when I run high I am carrying more glucose in my blood and it slows down my brain function so I tend to react on a short fuse. Also if he's thinking about his diabetes alot then it may cause him to react too, obviously talk things through and hopefully he can explain why he feels this way and what could be causing him to react.