Unfortunately realisation might be setting in that this isn’t just a temporary situation and he doesn’t want it - but of course not giving him the insulin isn’t an option so it’s a horrible situation to be in! Is it injections in the buttocks which particularly upset him? Could you do them somewhere else instead? My daughter has always flat out refused anything going near her buttocks! Trophywench has a good point too, I believe Lantus can sting a bit when it’s injected, in which case ask your medical team if you could switch to a different long acting insulin.
We had an issue 3 months in with a cannula which didn’t insert properly and I had to inflict pain in order to remove it, with the result that daughter was then terrified of cannula changes and we had to chase her round the house to get them done, and there were many tears (ours as well as hers!). We tried everything, reasoning, being stern, bribery etc etc but eventually after about 4 months she suddenly realised that the whole procedure would be much less stressful if she just let us get it over and done with, and then discovered that because she was less tense it hardly hurt at all anyway, and we’ve never looked back. I feel for you and hope you can find a solution soon!