I think there's some concern about skipping his insulin shot entirely. You could give less than 0.5u - look at the New to the Group sticky to find pictures of the small insulin doses. But with DKA so recent in his past, and now him seeming off and not eating normally, it seems risky to skip the insulin entirely.
Remember the ketone recipe is not enough insulin + not enough food + infection/inflammation = ketones.
However if he's not eating at all, that's also a problem. I'm with Dyana, I think a vet consult is going to be a smart call if you can't get him to eat at least 50% of what you'd consider a normal volume of dinner. You can try to assist-feed - many people do that at some point. Can you at least phone the vet for suggestions?
There are some tricks to get kitties to eat. Here are a few off the top of my head. If you haven't tried them, that might be a good idea.
Warm the food up - it will smell more and may be more enticing.
Sprinkle on parmesan cheese, oregano (related to catnip), catnip, or tuna juice (make sure there are no onions/garlic on the ingredients)
Make little meatballs of the food
"Prime the pump" by putting food in his mouth with your finger. Sometimes getting them started will help them get more interested.
sprinkle on the crushed dry food, fortiflora (if you have any)
get some rotisserie chicken - most cats find that pretty irresistible. Lunchmeat might be something else tempting.
Add water to anything he'll eat to help him get hydrated.
That's off the top of my head - hopefully others will add some tricks.
I'm running out for the evening but am sending Webster some "eat your fuds" vibes.