Sorry about your bird passing, it is so much harder when you get attached like we do to our special birds. Find another young bird or a pair, they bring so much enjoyment to our lives to not have them around.
Seeing that your boys poo is bright yellow it is a pretty sure sign that he has blackhead. I would follow the prescription you reposted in post #23 perhaps without the corid. Your vet should be able to do a fecal float exam to determine what worms, cocci, or blackhead protozoa are present. It...