Taking out some sunflowers from the mix is not going to do it. They cannot eat ANY sunflower seed - not a single one. Besides, if you want a bird to gain weight, you certainly do not feed it sunflower seeds, you feed it millet - which is what this species of bird can eat as long as it's measured (which means no seeds for breakfast and all day eating).
Let's try this again. Alexandrines are mainly fruit eaters. They do eat seeds but the seeds they eat are mostly what we call 'green seeds' which are the seeds inside the fruit (and this means the bird must eat a goodly part of the fruit before it even reaches the seeds). They cannot eat any animal food (meat, eggs, cheese, yoghurt, etc) and they cannot be free-fed any protein food (seeds, nuts, pellets, avicakes, nutriberries, etc). They need large portions of fresh fruits, leafy greens and veggies (different ones each day of the week) and low protein seeds (hence, the budgie mix which is made of grass seeds -low in protein and low in fat). And, because this bird is still a baby and it has not been weaned properly, you need to feed it soft foods (polenta/couscous/pastina/oatmeal/Iris oatmeal/etc mixed with baby food jars of fruit or veggies but NOT spinach and very little kale or gloop -you will find recipes in the diet section) served warm and fresh twice a day (breakfast AND dinner). And because the bird is too skinny, feed it also simple carbs (which is not something that one would normally feed a bird) like cooked white rice and birdy bread.