Hi Hendy, your baby should not look fluffed after feeding... at all. At this point your little budgie should be a very active little bird. The current photo you attached on this post still shows your budgie very fluffed. It's very sick and really, really needs help by a vet. You mentioned when you open your box the bird climbs out on your hand. What exactly do you mean by box? If when you bought this bird at 2 weeks of age you weren't keeping it in a warm container at the proper temperature this could have been the beginning of poor management of the baby along with not changing the nest material as you said every 5-6 days. We do need to know exactly what the formula is that you've been feeding. Do you make it yourself or is it a commercial brand of formula made for birds?
To be quite honest, and I would love to be wrong, but without any medical treatment by a vet this young bird is not going to survive much longer. It's obvious it is not doing well and can be a slow painful death for it. At the very least you should be keeping this bird warm at 85 degrees Fahrenheit or 30 degrees Celsius. You may need to place a heating pad on it's lowest temperature under the container you keep the baby in but only under one half of the underside of the container so that if it gets to warm it can move away from the heat source.