It's easy to refer to parrots in the same way as chickens if that is what we are used to. And in many ways they are similar... and in many ways, they are not! I know this is probably the last thing you want to hear, but if his droppings are collecting on his vent feathers, or if it's other discharge, he really should be seen by a veterinarian. These guys have the potential to live 80+ years and usually when a parrot displays signs of illness (irregular droppings and/or not keeping himself free of them are bad signs) it can be pretty serious. Possibilities include, but are not limited to: yeast infection in either the crop or lower guts, bacterial infection, malnutrition/vitamin deficiency, liver problems, kidney problems, avian diabetes (not quite the same as mammal diabetes), or... he just is very messy for some reason.
Can you get a photo of his rump so we can see how bad it is?
Is there any chance that it's a prolapsed vent?
What has his diet been like the last 26 years? I understand if you don't know... but if you don't, it's all the more reason to get a blood panel run on him to see if there is something he needs (or got too much of-- amazons are extremely prone to obesity and fatty liver disease).
As for working with him, it can be done but it will require a lot of patience on your part.
Here is some reading to get you started:
http://www.rationalparrot.com/biting.html
The bottom line is that you will need to make coming to you and being handled a rewarding experience for him-- or he will always hate it. After all, why would you go interact with a person you don't know well, or trust, and in the past has done things that you hated? Unless that person earned your trust and gave you nice things when you went to see them!
This can take a lot of time though. Some parrots trust faster than others. The person that wrote the above article had a wild caught
amazon and I think it took her the better part of a year to get the bird to step onto her hand without biting, and to trust her.
Of course it's not your fault that you had to catch him before... we all need to do less than desirable things with our birds sometimes when the need arises. My parrot hated having his blood drawn but it was done for his health, and is done rarely, so it's one of those 'necessary evils'.
You may need to catch him the hard way again to take him to the vet, I'm afraid, if you take him soon (which I recommend). Sometimes this is easier done if you can dim the lighting in the room (not dark-- don't want him to think a predator is trying to eat him in the dark). /Get out your gloves-- if he is gonna bite, he will bite you with gloves on or not at this point, so might as well protect your hands. See if you can get him to step up onto a dowel or large branch for you to get him into a carrier. If not, dedicate one of your least favorite, full size or larger bath towels to become a parrot towel. I know catching him this way is traumatic for you both but if he is unwell it might save or prolong his life, or at least his quality of life. Just be sure when toweling him to wrap firmly but not tightly, and to be careful of his wings.
