Sounds like he's reverted to needing hand-fed due to him being sick. You can syringe feed him formula or try soft mushy foods like mashed sweet or white potatoes (cook them first). Soaked pellets. Just soak them a few minutes in hot water, long enough to get them partly soft. Drain off the excess water. Bananas, etc. I'd place a bowl (I got a heavy crock type cat dish for JaKhu) on the cage floor with mushy foods for him to eat. Once he's over his meds & feeling much better try mixed veggies. I pop them in the microwave, with a little water on, for about 5 minutes. Drain, rinse with cold water to cool to room temp, drain. I do enough to feed 3 parrots for 2-3 days. Drain & put a heaping tablespoon full in a dish. Keep in mind parrots do not eat the skins of veggies. They actually will peel a pea, lima bean, corn, grape, etc. Keep pellets in his cage at all times.
Young macaws need to learn how to perch & how to eat different foods. To them seeds, nuts, chunk of a veggie is some foreign object. Their parents fed them regurgitated mush. You fake eating shows them the food is safe to eat. They will drop a lot, as they also need to learn to use their beak & feet to hold & eat the food. A great toy, that teaches them how to use those big feet, is a rubber dog ball with slits or openings in it. One bigger than a golf ball, but no larger than say a baseball. JaKhu would pick hers up & wave it around, bite it & throw it. Eventually she destroyed it.
My B&G Macaw (JaKhu) was pretty much weaned when I got her at 4 months of age. Parrot store/breeder told me no in-shell nuts yet, as her beak needed to grow more. Generally by the time they are weaned, or close to it, they are more interested in playing than eating. Being sick or in the weaning stage they will loose some weight. During molt they may also cut back on food or increase eating. Molting can be painful. They need a little higher fat in their diet, so a few nuts are a good option.