Welcome to the forum
What should I hand feed the cockatiel?
How/when do I start the weaning process?
Just with these questions, it may be best to speak with the breeder and ask if you can pick up the bird once he is weaned.
It is safer and won´t affect the relationship between you and the bird.
That´s a good link posted in prev. comment, I highly recommend you read it.
Where I live, breeders give you the food (formula) the chicks are eating. Just remember the formula must be at the perfect temp, too hot will burn the bird, too cold can give them sour crop/yeast infection. Also it is important that you feed correctly to avoid food entering the windpipe, causing the bird to aspirate.
Sorry I can´t help you on dates etc. I have gifted 3 of my lovebirds chicks to a couple of friends, and none left my sight before 6 weeks old.
I weaned 7/7 of my chicks with millet with no issues. I am now on the 8th, he is weaning fine on millet, I tried sprouts but he wouldn´t take.
Others like to use soaked pellets, blueberries, mashed sweet potato, broccoli, things like that.