You really ought to have quarantined your new bird
Quarantine IS Necessary! - Talk Budgies Forums
Your new bird could potentially have already passed an infection on to your new bird. Though just because an infection COULD have been passed on, it doesnít necessarily mean that it has, so please, put your new bird in a separate cage in a separate room now.
Even a cat/dog vet can help. They should still have the ability to test droppings for infection and to look up and prescribe relevant antibiotics. They can contact and take advice from other vets.
What can you do?
1. Prevent your older bird from also becoming ill, by starting quarantine.
2. Reduce any extra stress in your new bird by covering the top and 3 sides of the cage, leaving it alone other than necessary food and cleaning changes and keeping some background noise (radio on).
3. You could try adding guardian angel to help restore electrolytes lost via diarrhoea. https://www.talkbudgies.com/your-budg...te-recipe.html
The above points will only make him more comfortable, they wonít kill an infection and wonít cure disease, the best advice is unfortunately what you donít want to hear.