HELP...MY BLUE N GOLD MACAW IS SICK?

HE'S RECENTLY LOST ALOT OF WEIGHT, HIS MATE PED AWAY N JULY (FOR NO APPRENT REASON OTHER THAN SHE WAS PRETTY OLD), BUT HE IS SUFFERING FROM DIARRHIA, HE DOES HAVE BLOOD N HIS STOOL BUT THE VET RAN TESTS AND SAID HE COULDNT FIND A PROBLEM. HE REFERRED ME TO A UNIVERSITY RESEARCH PROGRAM BUT MY BIRD DIDNT QUALIFY FOR THEM TO DO ANY SORT OF DIAGNOSTICS ON HIM. I TOOK HIM TO ANOTHER VET AND IT WAS THE SAME ANSWER. NO APPARENT CAUSE FOR BLOODY STOOL, LOSS OF WEIGHT AND DIARRHEA...IM BAFFLED. HE IS WORSE TODAY THAN EVER. THIS HAS BEEN GOIN ON FOR QUITE SOME TIME AND CANT FIND HELP. I FEED HIM NUTS, SEEDS, VEGGIES, FRUITS, DAIRY, AND EVERYTHING I EAT...PLEASE HELP. I WANNA STOP THE POOPS SO HE CAN RETAIN SOME NUTRITION.