? What's the best time for a snack?

As he's so underweight and constantly hungry, I want to give my cat a small pure protein snack(eg. plain white fish or plain poache d chicken) between his main 12 hourly feeds.
What would be the best time after his shot and breakfast/supper for this?

 

Many of us free feed our cats rather than feeding only on a 12 hour schedule. Then we pick up the food 2 hours prior to the test/main meal/shoot insulin. If your cat is underweight and constantly hungry, I would feed more often.