Great job in getting vet check up!
What is his weight?
Is this recent? Have you had him all six years?
Do you have other birds.
Does he fly?
If he flying does he loose steam fast?
How do his poops look? Normal is formed fecal part , a creamy white part and just a little liquid part. Abnormal is a lot of liquid part and stringing urates . Extra liquid after veggies or fruit is ok tho.
I don't get blood work on my GCC, borderline to small, would do if she was really sick and we couldn't figure out otherwise. In the past some could have bleeding conure syndrome , after a blood draw they would keep bleeding have trouble clotting. I don't hear about this much anymore...but because I had , I've remained hesitant with my GCC.
Somethings that are possible, but not sure how likely.
Yeast infection, my GCC dropped weight with yeast overgrowth, no symptoms , other than I could tell she smelled yeasty. Didn't have bubbles in poop . She went down to 62 or 64 grams. After treatment went back to her 74 that she is stable at..
Chronic low grade Chlamydia , can present as only low weight. But often at least have bright green urates as well.
Internal parasite like tapeworm or round worms, extremely rare in our captive parrots , especially if you live in USA . Unless kept with finches , exposed to outside birds, fed some insects or critters that are part of the lifecycle.
Cancer , pretty low chance of this, reproductive being most common type of this rare thing for birds to have.
This is not everything, but what came to mind.
Can be as Mizzly shared, a normal low weight
I have a dog that has high metabolism and very thin. She is 8 now, when she was younger I had worked up for all kinds of things , and repeated work up again a couple of years later. The vet couldn't find a reason, thyroid checked, blood sugar checked did a whole blood sugar testing protocol, cushing test , blood work, urine, fecal, x ray, ultrasound, treated for parasite even tho none ever found. We really tried! I ended up having one litter of puppies with her and kept one. He is the same a very thin trim dude , but happy and healthy! I have to feed them a higher fat and calories diet to keep them thin but not let get skinny or boney