Your major issue is his regular daily diet...Millet is not a staple food that should be his regular meal, it's only an occasional treat...It is full of nothing but fat and very little protein. So he doesn't really have a 'disease' at all, he's obese, he probably has Fatty Liver Disease which will end his life early, and he is definitely malnourished, because millet is not providing him the proper vitamins and minerals or the protein that he needs to stay healthy. His immune system is no doubt suffering too...
Do you have access to a commercial Budgie seed-mix that you can buy in a pet shop or order online? Because that's what he needs to be eating...Most commercial Budgie seed-mixes contain a little bit of millet seed, but different types, along with all kinds of other seeds, grains, legumes, etc. that will provide him with the proper vitamins, minerals, protein, etc. that he needs to be properly nourished. And it won't be nearly as fatty as only feeding him millet seed/sprays.
As far as fresh veggies, 'lettuce' must be the dark, leafy varieties and not Iceberg or any other :"watery" lettuces, as they contain no nutrition at all and are just water...He should be getting fresh greens every single day, if he'll eat lettuce then he'll eat all of the dark, leafy greens such as Collard Greens, Mustard Greens, Turnip Greens, Arugula/Rocket, Bok Choy, Pak Choy, Chards, Broccoli Rab, etc. These are the types of greens he needs to be eating every single day, as they contain all kinds of vitamins, minerals, amino acids, etc. Lettuce, even the dark, leafy kinds like Romaine or Red Leaf do not contain much nutrition, not like the other dark green, leafy Greens...And then also fresh Veggies chopped-up into tiny pieces, such as Broccoli (They love the flowers picked off), Green Beans, Pea Pods/Peas, Sweet Potatoes, all kinds of Squash like Spaghetti, Butternut, Acorn, Yellow, etc., Zucchini, Carrot, all colors of Bell Peppers (they love these), etc. They need to be able to get a variety of dark, leafy Greens and fresh Veggies every single day (no canned or frozen veggies)...As far as fresh Fruit goes, it is all full of sugar, which will turn into fat and be stored in his liver just like the fatty Millet and Sunflower Seeds are, so fresh Fruit should only be given occasionally, maybe 1-2 times a week, and in very small portions...And no Citrus Fruits or Tomatoes, as the high Vitamin C content makes them very sick, and no Avocado or Onions, Leeks, or Chives, as they are all toxic to birds...
I would assume that the reason he can't fly is due to 2 different reasons, the first being his weight (though I can't see how obese he is, and this still really isn't going to cause him to not be able to fly), but more so the severe malnutrition, lack of energy, and the lack of much protein in his diet has probably caused his muscles to atrophy, so he has no strength to be able to fly...
Another issue is the fatty, enlarged Liver he has to have...What happens when a bird's Liver enlarges is that it takes up all the empty space in their abdomens, and it actually pushes up on their Keel Bones; the Keel Bone is that large, flat bony plate that covers their entire chest/abdomen and that you can feel the pointy blade of going down the middle of their bellies...When the Liver gets very large it pushes their Keel Bone upward and outward, this is called "Sternal Lift". And when they start to experience "Sternal Lift", then all of their other internal organs like their Spleen, Stomach, Pancreas, as well as their Abdominal Air Sac all shift their positions inside of their abdomens, and so their weight-distribution is all out of balance, causing them to not be able to fly. So while yes, he does need to lose some weight, it's actually more important that the fat in and around his Liver decrease, and his Liver will then shrink to it's normal size, which will allow his Keel Bone to come down to it's correct position along with all of his other organs, and then his weight-distribution will back in-balance, and he'll be able to fly again...So it's not so much that he's overweight or obese that is causing him not to be able to fly, it's his enlarged, fatty Liver that's doing it, along with some likely muscle atrophy due to not enough protein in his diet...
You need to get him on a regular, daily diet that is 50% a well-rounded, commercial seed-mix for Budgies and then the other 50% needs to be only fresh Veggies and dark, leafy Greens, not Lettuces...You will need to chop-up the fresh Veggies into small, tiny bits for him, as Budgies won't usually eat them otherwise, and you'll have to try different fresh Veggies and Greens to see which ones he loves the most...
I would advise you to completely stop ALL millet sprays and any other treats or people food, especially no people food if he gets any, and no Fruit at all for a good couple of months, and only feed him a varied Budgie Seed-Mix in his bowl at all times, and then another bowl of fresh Veggies and Greens twice a day, once in the morning and once in the late afternoon/early evening...And that's it, no other foods, junky seeds like Sunflower Seeds (absolutely none of those, and most Budgie seed-mixes don't contain Sunflower Seeds so that's good), no Nuts of any kind, no Dried Corn or any Corn for that matter, just the commercial Budgie Seed-Mix you buy in a pet shop and lots of fresh Veggies and dark, leafy Greens...
If you follow this diet strictly, he'll start losing weight and his Liver will start to also lose fat and shrink in size, and you should see a much healthier, happier, smaller Budgie who will gradually regain their ability to fly again...But if you keep feeding him the diet he's been on, he's not going to live much longer...I'd also see if you have access to Milk Thistle in a supplement form or powder form, and I'd be sprinkling a bit of it on his new seed-mix every single day, as Milk Thistle helps very much to regenerate Liver tissue...
