I'm not really completely sure that his mother is visual pied since her markings are extremely light, but so far all their babies have appeared to be visual pied not split. It IS possible to have a split pied bird with a lot of yellow feathers on the body. But it's apparently rare to have a split pied bird with even a single yellow flight feather, let alone a whole bunch of them. I think he's visual pied, and the mutation just simply failed to mess up the coloring in the face area the way it usually does. The pied mutation doesn't follow the rules in a lot of ways, including its own rules sometimes.

what's the difference between pied and split? I know what pied means visually, but that's it.
Is split when they appear pied but carry genes for a different mutation?

Split means carrier. You carry the gene but don't have it yourself.

For example: the parents had albino children but none of the parents were albino. That means that both parents have the albino gene. This is talking about humans but they same concept can be applied for cockatiels.

Yes, B has it right. A bird is split when it has one copy of a particular gene, but it takes two copies (one from each parent) to actually be visual.