All his tests came back negative. Maybe it is diet....
I'm so glad to hear this, Ann! The test, unfortunately, is not conclusive for ABV or, by extension, PDD, but most other diseases can indeed be ruled out this way. Though I don't know what else you tested for other than ABV and PBFD.
It may very well be diet, as you've suspected. Especially considering that a large proportion of his food consists of sunflower seeds. I'd say just stay consistent with your attempts to transition him to a healthier diet. Some birds are more stubborn than others, but it's largely a matter of trying different things. Keep throwing stuff at the wall and see what sticks, you know?
As for the lurking possibility of ABV/PDD, unless he shows further signs (i.e. more undigested food in droppings or neurological quirks such as diminished coordination or even seemingly compromised eyesight) just continue to approach as though it is a diet issue.
If you do notice anything that could be a symptom, another route to take is to have x-rays taken. An enlarged proventriculus would be a rather definitive indicator of the presence of PDD. (Though not all cases of PDD will present this indicator, a very significant amount does.)
Please continue to keep us updated. I'm really hoping that it really is just a matter of diet.