A bit of lemon juice in drinking water will provide electrolytes. About a teaspoon or less per cup of water -- more wouldn't harm the bird but might cause the bird to avoid drinking the water.
There are a lot of possible causes for symptoms of central nervous system issues such as being unable to stand or fly. Trauma from impact is one, many viral and bacterial infections can also be the cause and the bird can survive those infections and recover fully -- provided that the bird is in a safe place, and has food and water available. At times the bird may not be able to eat or drink on its own, and would benefit from assistance. It the bird is having trouble holding its head up, it would be best not to leave a deep dish of water with the bird, but rather use a very shallow dish.
Various foods are naturally antibiotic and/or antiviral which will assist the bird's immune system in fighting illness. Some examples are seeds such as cumin, fennel, peas, lentils, anise, and rosemary. Raw apple cider vinegar also helps and can be added to the bird's drinking water at about one teaspoon per quart/liter of water, and like lemon juice, more would be ok for the bird to drink as long as it doesn't cause the bird to reject the water completely. Probiotics may also help, and some can be added to the diet by including occasional tiny amounts of yogurt with "live cultures". Greek yogurt has less lactose which birds don't digest well, so it would be better then regular yogurt.
The diets of feral or wild pigeons and doves are frequently deficient in vitamin A. A few daily bits of chopped carrot or bell/chili pepper will provide plenty of vitamin A in a safe form. Some other vitamin and mineral deficiencies can cause symptoms such as weakness and inability to stand, walk, or fly -- insufficient B vitamins are one, and the combination of vitamin D, calcium, and phosphorus is another. All of those can be found in hard-boiled chicken eggs (all in the yolk, the whites are mostly protein.) Feeding the bird crumbled hard-boiled egg yolks along with seeds would ensure that the bird is getting some of all needed vitamins and minerals.
If the weather in your location is below 90 degrees Fahrenheit, provide additional warmth for the bird such as by wrapping a hot water bottle in a towel and placing it near the bird.
