OK, as long as you don't see any other symptom like lethargy, sleeping too much during the day (like right after breakfast, for example), fluffing up, discharge from anywhere, vomiting, diarrhea or too much urine, weakness in flight, etc. don't worry too much because it's pretty common for babies to lose a bit of weight once they start weaning in earnest and flying more. Are you making the formula thicker? Because they do not like the liquidy kind they get when they are tiny babies as they grow up. Try making it thicker, using a larger syringe or a spoon and make sure it's nice and warm. Again, check the amount of poop. There should be a very large poop early in the morning and lots of little poops during the day (they usually poop a lot in the morning, kind of stop at noon, and poop less often in the pm). Check the way it looks and smells. Healthy poop for a baby is more 'liquidy' than adults but there should be distinct feces after the first morning one. And it should not smell -when it does and it's sour, it means a fungal infection. Smell his beak, does it smell sour? Do you see him eating gloop or whatever soft food you are using? Are you keeping him at a solar schedule and feeding him at dawn and dusk? These are the times that he has been 'programmed' to eat as diurnal parrots are crepuscular feeders.
Try giving it some white rice, millet and white bread (these are all fattening). Put some aloe vera juice (not gel and not from the whole leaf, only juice from the inner filet) and some organic apple cider vinegar with the mother in his water (do 3/4 water -use good one, not from the tap- 1/4 aloe vera juice and one tsp of vinegar in a pint of water). These will help if he has sour crop or some sort of yeast infection somewhere in his digestive tract.
You can also get some wide spectrum antibiotic and give him a week of it. If he has an infection, it will take care of it but antibiotics also fatten them (that's why meat producers used to use them all the time in the animals for slaughter). Try this:
https://allbirdproducts.com/collections ... vet-liquid It's a good wide spectrum and very well accepted by birds, easy to get the right dosage (see the instructions below) and causes no stress to the bird because you put it in the water. You take out the 'old' water at night when the bird is already asleep and, at dawn (and this HAS to be done at dawn!), put out the fresh medicated water. You will see the bird drinking a more or less pre-determined number of sips - and these sips will amount to the right dosage.