If you are dealing with a picky eater, try to give them smaller portions at each meal. By doing this, they won't feel overwhelmed with the amount of food in front of them and if they are hungry, they will be more inclined to eat their entire meal since it is not a huge serving size.
A great nutritional tip is to check in with your doctor regularly. By checking in with your doctor regularly, you'll become aware of any health changes and you'll know if your diet is lacking in any way. You're taking a gamble if you don't visit your doctor.