I'm very sorry your little Susu is not feeling well.
As a general rule, it's not advisable to give medicine/antibiotics when we don't know which illness our birds have.
Did you just see the change in Susu today? Did he managed to eat something today?
How is Susu's breathing, do you notice a prominent tail bob? Does he sneeze and do you see any discharge coming from the cere?
Do Susu's poops look normal in colour, shape and consistency?
For now you can cover the cage on 3 sides to keep him warm and you can offer him some spray millet soaked in electrolyte solution, this will hopefully give him a boost of energy.
Here is a link that can be helpful:
If you really aren't able to consult with an avian specialist vet, then you can try to get advice from one online. You will have to be able to describe all symptoms for a better diagnosis and treatment plan.
I'm moving your thread to the Emergency Room section of Talk Budgies.