Are you using frozen peas which have been thawed in hot water and served at body temp? I think you could try opening his beak and placing each pea in the back of his throat, the close his beak til he swallows.
Did you see my post #9? Click on '50'. It illustrates a small plastic bottle with the top opening covered with a latex glove and a small slit cut for the beak?
The bottle is what I have used for feeding my own homemade baby pigeon gruel. It is made from chicken pellets softened in warm water. I then put them in a blender with some more warm water and blend til smooth. Optional add ins are tsp peanut butter and nutritional or brewers yeast.
The bottle mimics the parent pigeon mouth where baby inserts beak. It works better for me than a bag although I have heard of people succeeding with bags too.
Best of luck to you and Geoffrey! @CCUK - any advice on finding baby bird formula in UK??