At 3 weeks you can start adding some small seeds to the formula. You can leave a bowl of seeds with him and encourage him to peck at the seeds by playing with it using your fingers. They view your fingers as the food source and will copy what your fingers are doing. Do the same with a small bowl of water.
When you are weaning him, still feed him in the mornings and try and encourage him to eat during the day. Sometimes they learn quickly, others take time to learn. He might only start eating one type of seed first, so always make sure those are available. Eventually he will move on to the other seeds. If you think he has not eaten enough during the day, then you can feed again at night.