What kind of bird is it? What have you been feeding it?
Some species, especially passerines like sparrows and song birds, feed their young a pure insect diet. They choose only the right kinds of healthy insects, and will often remove sharp bits that could harm their offspring. If the bird is one of these types, it will need a high-protein content food like hard-boiled egg yolks mixed into a somewhat loose but dough-like, multigrain baby cereal (50:50 yolk to cereal). Kaytee Exact hand-rearing baby formula will be best, but depending on species, you may need to adjust carbohydrate, protein, and fat ratios.
The food should adequately provide all the necessary water/moisture content the baby bird needs, but not be runny or too watery. Never try to force water, or runny food, into the throat or crop, they can easily or accidentally inhale it, choke, aspirate or drown.
Most altricial birds are fed usually every four hours or less by the parent, depending on species and age. Species like doves, may feed nestlings crop milk during the night, but most others will just ensure a large, last-minute meal and an early large breakfast at first light.
Keep him/her warm. This is very important, and may be why your bird is having issues, aside from type of food. The baby bird will definitely need a very warm environment in order to properly digest meals, especially if they are small or very young. For small nestlings, with little feather down or feathers, they will need constant warmth at around 35-37°C.
Also, if the air is too dry (<30% relative humidity), provide a little air moisture by placing a damp, clean rag or washcloth nearby, and change daily. Proper air humidity is important, especially in very young or small birds.
Also, try not to overfeed them when there is undigested food still in the crop. You can easily see into the crop, as both the skin and the crop membrane is very thin and nearly transparent, especially in baby birds.
With proper humidity, warmth, and feeding he or she will have a very good chance in thriving, granted there is no health or illness concerns.
If you suspect a fall from the nest, especially if the bird doesn't have any flight feathers or isn't fledging/flying, or could have had a far fall on hard ground - the little bird may have injuries. Check especially the legs for injuries or broken bones. Check the body skin for bruising or hematomas, especially on the underside. Ensure there is no popping or light clicking sounds while the bird is breathing. Any of these will warrant emergency urgent care and epert treatment.
You will need to make an artificial nest out of a small bowl or a rolled-up old sock with padding and grip. This is so it's little legs don't splay out on either side of it's body, but are kept underneath. It may cause permanent leg disformity if left splayed outwards for a few days or more, depending on it's age, development and strength.
If you live in the US, and the bird isn't a native species like an European starling, Eurasian/English House sparrow, an Eurasian Collared dove, or a feral (Rock dove) pigeon (unlikely it is a feral pigeon, as they don't nest in trees), you can call a vet and/or find the nearest wildlife rescue/rehabilitation center (or call animal control), as other native species are protected by law in the US. A vet can advise and give people permission to pick up and rescue a wild species on a case-by-case basis. You could get into trouble and be fined $10,000 even for picking up a wild bird in the U.S.. Most other countries have similar laws on native or migratory birds. Any licensed or accredited vet, or rehabilitation or rescue organization will be able to care for him or her and be able to carefully raise it and sucessfully release it, without having the bird imprint on people or loose it's fear of humans.