You can get fish mox at just about anywhere that has fish - aside from petco. Check locally because you'll want to get this to him as quickly as possible. If you've already ordered online no sweat, these tablets if stored properly can last a really long time. Always good to have plenty on hand and it is becoming increasingly difficult to find.
It goes by the name aqua-mox, fish-mox, etc. Just ask the fish store if they have amoxicillin for fish.
Most come in a 500mg tablet. Dosage for an adult bird is 250mg.
1/2 tablet 2x per day, or 250mg.
The last stuff I picked up in a hurry recently was 800mg/tablet and I'm glad i caught the difference.