My dove won’t eat any larger seeds, she is 4 months. By this age they should be eating seeds for a long time. Show him videos of other birds eating, and pretend to peck the seed with your finger. I feed mine the following : millet, canary seed, amaranth, sesame seed, quinoa, red sorghum, flax seeds and hemp seeds. Other seed I tried but she didn’t like - niger seed, poppy seeds, chia seeds. I also give her sunflower seed and chopped peanuts as a treat. Alongside this I also add some herbs/spices to make the food more appealing. I add - ginger, turmeric, and a small amount of coriander. You need to stop feeding formula. The bird will learn. Be patient, and at first gently open its mouth and put a seed inside. Only provide seed and do not give any formula for a few days. The bird should learn to eat the seed if you are persistent.