Cockatiels are criers. But many other species will do the same thing during weaning. At the age you listed, he's probably in the weaning process and may or may not be ready to wean completely. I think sometimes they cry if they are a little insecure. Also, the loudest mouth get's fed first, so it may be a survival thing. In his mind, he may default into cry mode when in doubt about what to do.
Is he flying yet? Weaning usually kicks off in a major way a few days or a week or so after they start flying. Before flying, they want to loose weight and may cry intermittently and then only take a little bit of food. I usually continue to at least offer a little bit of food after they start refusing a meal or only eating a little bit. Weaning babies are very on again/off again about food. Sometimes, its good to just let them out to fly around and then after they've had their flight practice, they will be ready to eat a little bit.
It also helps to just freshen the food. You don't necessarily have to prepare a grand meal, just add a little more seed, a little more pellets or a little more veggies to the bowl to get him interested in trying to eat on his own. If I have a baby that cries, and then only wants a minuscule amount of food, I will either freshen the adult food or give some soft veggies to eat. A cockatiel may or may not be interested in cooked soft sweet potato with beans and rice which is what I give all my amazon, quaker and parrotlet babies as a comfort food. If I had a cockatiel, I'd probably try putting some fresh sprouted seeds in his bowl in order to direct that feeding impulse into appropriate adult foraging behavior. It also helps to give a partial hand-feeding meal (whatever they will eat without persuasion) and then put them in their cage with some fresh food and walk away so you are out of sight. If there is no mommy to cry at, but a bunch of interesting adult foods to investigate, they will often start eating their grown up food. Especially if they've had a couple cc's of formula and the edge of hunger is off so they feel a little more confident to investigate other things rather than sit and cry.