Hi supporter, welcome to the forum
🙂 Does your partner drive? If so, then what he is doing (or rather not doing) is illegal - he MUST test prior to driving to ensure he is safe to do so. If he were to have an accident, even if it was not his fault, then he would probably lose his license as he would not be able to provide proof of testing. Also, if he is not testing prior to injecting his insulin then how does he know how much to inject? Does he have many hypos? If he's not having hypos then it is likely that his levels are generally running higher than they need to be. Does he attend his appointments for his regular checks?
Well, I'm sure you are already aware of these difficulties. The problem is that someone who has been diagnosed for so long is probably very 'possessive' about his diabetes, and does not feel that anyone can tell him anything about how to deal with it. This is a big thing to overcome with the danger of being a 'nag' or accused of interfering

You might find the following useful, it is a guide to 'Diabetes Etiquette':
https://www.accu-chek.com/documents/resource-center/etiquetteonepage.pdf
It might also be a good idea to contact the Diabetes UK Careline service for advice - I'm sure they get enquiries of this nature all the time:
http://www.diabetes.org.uk/Guide-to-diabetes/Teens/Help-and-support/We-can-help/Careline/
People do suffer from 'diabetes burnout', where care of their diabetes becomes overwhelming, but I suspect that your partner has just become complacent due to his long time of dealing with it. Perhaps you can persuade him to get a 'refresher' appointment with his consultant or DSN, just so he can find out where he stands with everything? It may be that he has been very lucky and all is fine - although this would still not mean that it is OK to skip testing before driving.
Good luck, let us know if you make any progress!
🙂