It is highly unlikely that there is something stuck in his crop or throat - that is pretty darned uncommon and would not explain his other symptoms. The regurgitation is almost certainly due to the antibiotic/sulfa. Continue the antibiotics, just give them to him diluted or small doses or after soft foods or all of the above. Antibiotics do generally work quickly (2-4 days), but not this quick and didn't you say he is better?
If it were my bird, I would religiously continue the supportive care (heat and hydration and quiet), the antibiotics and cut out the seeds. Replace the seeds with 100% soft foods.
Just continue to be the good little nurse and keep him as stable as possible until you see your vet. Write all of your questions down and also write down any specfic things you want the Dr. to know. Such as what he's been eating, symptoms reactions to meds etc. When you are upset one has a tendency to forget things. Also, make the Dr. explain what he finds until you understand.
ML