OK, I just barely turned 8. As my mum said years later, I had to deal with it on my own at some point.
I was diagnosed at exactly the same age, but luckily my mother splashed out and bought disposable plastic syringes instead of the NHS glass reusable ones that she had been using. I remember being very proud that the hospital did not even get me to practice on oranges first but I was able to go straight to injecting myself. But 8 is very different to 5 in terms of rationality and understanding.
However, back on topic.
@Eddie12 has a point that injecting yourself is less painful, as you can move the needle if you hit a bad spot. Even after 50 years of self injections, I have to look away if a nurse gives me a flu jab or takes blood.
Have you tried using a freestyle libre? Not sure whether this is advisable for young children, but it could cut down the need for finger pricking significantly.
There are usually a few children every year who die of diabetic ketoacidis before they get a diabetic diagnosis and life saving insulin. I sadly suspect that during the COVID crisis this figure will go up. (This is not a criticism of your understandable desire to not traumatise your son, just an observation to try to keep things in perspective.)
I'm afraid I think that
@lucylocket61 's friend is right, you can't negotiate on insulin injections, but agree that the rufus bear (supplied by JDRF) might help. They have a phone number, though I don't know whether it'll be manned in these covid times.
https://jdrf.org.uk/about-us/contact-us/
Good luck again. I cannot imagine how awful it must be to nurse a child through the very early years of diabetes. Things really should get better when he is older.