I would try to get him used to you touching the places that you will have 2 wen putting on the harness.
Under the wings ect. Remember treat it as a game and have a bowl of his fave treats nearby!

Then get him used to the harness by gradually introducing it to him. Hang it somewhere outside his cage. Then in the cage, then play with it outside the cage with him...
Then start to loop it around his neck and reward LOADS!
Then when hez used to that even a bit, start to do the bit behind the wings as well. Then leave it on longer and longer, always in the house, reward ect.
Then put the leash on and walk him around the house on your sholder or hand. Then when hez used to that take him outside - and - hes harness trained!
Linzy 