Feral cats are not easy, and as others mentioned it's great that you can get close. My own girl, Friendly as she was ironically named, lived with Ron and I for 18 years but we could never touch her, we could only look at her and her beautiful eyes. Catching her for vet visits meant putting on long sleeves and gloves. Late in life she was diagnosed with hyperthyroidism and needed vetting every few months and it was terrible for her.
Since you can hold him, you may be able to give insulin, although home testing might not be an option. When I give shots to my kitties, even the new ones I adopt who don't know me well, I wait until they are eating and are concentrating on something else. I play with their fur, rub my hand on their side and very quickly pull up the skin and shoot. Since the needles are so tiny they almost always don't notice.
In time I would try to work with testing. Touch his ear a little, give him a freeze dried chicken treat for letting you do that. Only give the special treat when you touch his ear. Then in time, move on to using the lancet device without a lancet in it, just touch it to his ear, let him hear the click then give him treats. Try to work your way up to testing.
If you do change his food to canned only or raw and the folks here will be great at helping you find good food, he may not need insulin, but you won't know unless you can test him. For the time being you could try pee sticks that measure the glucose in urine if he will use a litter box. That would tell you if diet change alone was working. If he needed insulin the pee sticks aren't ideal because they don't tell you what his blood sugar is immediately, but instead what it is after hours of traveling through his body and kidneys.
If worst came to worst and he absolutely needed insulin and you could not give it, there are pills although they are not good for cats. They can cause liver damage. I haven't used them myself and they are not popular, but it is an option. It would be my last resort though. That is if you could get the pill into him. Hiding the pill in pill pockets or cream cheese might work. But again, I really have to emphasize that the pill would not be ideal, I'm only putting it out there so you know it's there. From Wiki about the pills:
Pills
Oral medications like Glipizide that stimulate the pancreas, promoting insulin release (or in some cases, reduce glucose production), are less and less used in cats,[14] and these drugs may be completely ineffective if the pancreas is not working. Worse, these drugs have been shown in some studies[15] to damage the pancreas further, reducing the chances of remission for cats. They have also been shown to cause liver damage. Many owners are reluctant to switch from pills to insulin injections, but the fear is unjustified; the difference in cost and convenience is minor (most cats are easier to inject than to pill), and injections are more effective in almost all cases.