So here is the update.
He is doing so much better. He is already gaining his weight back. He gained a gram and a half which doesn't seem a lot but it feels like it is as he was losing so much weight (he weighted 29 grams on Friday while my other budgie weights 43 grams). He has much more energy and eat more as well. There's also not that much liquid around his poop.
He gets broccoli and carrots everyday along with different veggies and fruits (spinach, corn, apples, cucumber, celery, green peas, peppers). I also added pellets to his seeds.
I gave him dandelion leaves as my vet suggested ( I forgot to mention it before)
My vet also suggested to give him watercress which I will once I find it in the shop
Also what do you mean dandelion roots? Do you mean roots as roots or something I should buy?
I will definitely look for milk thistle and add to his water once he finish with Baytril.
Thank you for the suggestions and replies.