It sounds like you are doing all you can.
I wanted to give a warning about burping. I’m into the probiotics stage with a hen that just had sour crop. Massaging and burping her made her crop go down and she would feel better. Then she threw up. Birds don’t throw up.
Make sure while you massage his crop that you are ready in case the burp brings up more then air. Head down butt up and try to get his mouth clear. The biggest concern is aspiration of the vomit.
I wanted to give a warning about burping. I’m into the probiotics stage with a hen that just had sour crop. Massaging and burping her made her crop go down and she would feel better. Then she threw up. Birds don’t throw up.
Make sure while you massage his crop that you are ready in case the burp brings up more then air. Head down butt up and try to get his mouth clear. The biggest concern is aspiration of the vomit.
but a lot of times it's the only way to get some answers unless you send the body to your state lab. 
) but I'm also quite practical, so becoming focussed and absorbed by what I find inside them helps to distract me from the grief. I recommend covering the bird's head with a sock or something once you have checked inside the beak for lesions or plaques. For me the head and eyes are what I identify with so once I have that covered it is just a chicken carcass. I don't use any special tools, scalpels etc are not necessary. I have a little pair of sharp scissors and a sharp kitchen knife and I work in my poly tunnel/greenhouse rather than dissect the bird in the house. The light is good in there, it is warmer than outside and if there is any infection I'm not bringing it in the house. I work on my potting bench on an old plastic feed bag. It helps to have a bucket of warm soapy water and an old towel so that you can rinse your hands when you need to and if you are going to take photos, which is always really useful if you need help figuring out what the problem is, having your phone/camera wrapped in a plastic bag means you don't have to keep taking gloves off to pick u[p your phone for a photo. Taking shots of the organs in situ and then removed enables us to have a better idea of what we are looking at. If there are abnormalities, it is sometimes difficult to ID an organ other than by it's location in relation to other organs.
