How to gain weight?

Hey there,
I was told by the vet that my budgie is underweight and needs to gain some, but every day since I got him hes always 40g, I had him on almost no seeds and pellets instead but ive been giving more seed now to try encourage a bit of weight gain, he always has veggies and millet and I make sure he has some of all three close to wherever hes perched when hes out of his cage.
I dont know how else to encourage him to eat than how I am (eating it, offering some, leaving some within reach) and he likes to try everything pretty much so far apart from fruit, he loves greens but Id like to have him eating something more substantial, and cant seem to get him to gain weight.
I think hes crossed with english budgies, hes about 7 and a half inches long from top to tail tip but its hard to measure him as hes suspicious of the tape, ive had him three weeks now an I think hes 6 months old.
I can kinda feel his keel, its not squidgy, hes only just started getting very keen on flying around and exercising the past week since he was taken to the vet and started on treatments for a respitory problem that we're still trying to diagnose, the medicine is working though, but his appetite seems the same..

Any tips for encouraging him to eat more/gain weight in a healthy way? Im worried he wasnt eating enough when he was unwell and wont have much of an appetite while on antibiotics for the next few weeks, I started to mix some probiotic powder in his food but it doesnt seem to bother him

here is a gallery of photos of him for size reference, the photos are mostly pre-vet an after starting on medicines.

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I wonder if his appetite will improve as he starts to exercise more? Also I think muscle weighs more than fat, so as he builds muscle from flying he'll put weight on. Don't forget if he's been fighting an infection he'll have used a lot of energy doing that. His appetite might be fine, he's just been using the energy to fight illness rather than put on weight. Give him a bit more time with exercise and in good health and see how he does.

I was just going to say the same thing, you don't want him to gain fat around his keel/chest/stomach, but rather muscle, and the only way for him to do that is to fly on a daily basis. And the muscle gain will cause a weight gain, as muscle is heavier than fat. So the best thing you can do is keep him on the pellet/seed mix/fresh veggie/fruit diet that you are, definitely keep him on the probiotics until he's been done with the antibiotic for at least a week (you don't want him to develop a fungal infection from the antibiotic killing-off the healthy bacteria in his GI Tract), and encourage him to fly around as much as possible daily. He'll quickly start building muscle around his keel bone.

Is your vet a Certified Avian Vet? Or are they simply an Exotic's Vet or General Vet? I ask because your vet should have been able to educate you about the healthy way for a bird to "gain weight". And just because you can feel his keel bone doesn't necessarily mean he's underweight, it simply depends on the distribution of his muscle mass around the keel bone. He doesn't look to me to have any English Budgie in him, meaning he's an American Budgie, which are a bit smaller than the English Budgies are, so he looks like a healthy American Budgie who just needs to keep eating as he has been (don't be surprised if his appetite picks-up a lot after he finishes the antibiotic), and start flying on a regular basis to build-up his muscles.

Yes, certified avian vet, please. If you dont have one, I hope you'll get one, and then follow their advice faithfully.

A personal note... the Rb has always been lean. MY CAV says he's just apparently an endomorph, and may be healthier for it. So if your bird remains a little skinny, it may be fine.

Good for you for reaching out.