The pet store seed mix also didn’t seem great (lots of millet, sunflower seeds, peanuts)
Is this the only mix available? What are your options at the store?
My current seed mix is sunflower seed heavy, so I skim a load off the top before using. Then a gently shake side to side and more sunflower seeds come to the top, and I skim them off again. I save them in a jar for rewards.
I don't know why there would be peanuts in your food though, you could do with a no peanut mix.
Along side your seeds, offer fresh veggies daily, some nice romaine lettuce leaves and little pieces of carrot, pepper, etc When you have one little bird, just cut a small piece off of whatever you are eating/preparing for lunch/dinner to give them. I have a tupper in the fridge so when I'm cooking dinner at night, I save a few pieces of veg for the birds the next day (I am not a big lunch eater). You can use frozen veg - take 2 or 3 peas out of the freezer and defrost for a few minutes. Or broccoli florets, green beans - they can all be used frozen and just defrost one or two pieces as you need them to bulk up meals. I buy the little mini peppers for the birds, and they get small berries too.
Then you could offfer a calcium/vit d 3 supplement to be given so they don't lack any vitamins. Don't use the supplement all the time (daily 24/7).
edit to add, if he doesn't take to eating the veggies right off the bat, keep trying. You can also sprout some seed, this is often a good way to introduce green foods.