"where do you guys learn all this?"

I'm not a CPA; I lurk here and learn from the others. I consult with small businesses and other entities, and I teach QuickBooks, which means I need to understand how these things should be tracked, which means I need to understand what is done from the beginning and what are the options and impacts and considerations. One of the CPAs here is better able to address hypothetical basis and gain as per your examples.

As for this (instead of an outright sale to start with): "What would Son's basis be in a year or two should he decide to sell his shares?"

From the perspective of what they actually did, you have the answer right in your topic, above, which is why I read these topics and how I learn about these subjects:
Son's basis in stock = 49% of father's basis immediately before transaction. <== sjrcpa

Because that is a gift of Basis minus $10. Again, as provided by sjrcpa:
The gift is FMV less $10. Son's initial basis in the stock is $10 + 49% of father's basis.

Because of the way they did this activity.

"it was not in any of my textbooks..."

"was" never matters. Textbooks are not a useful reference, after printing date. "Now" is what matters. You know the tax regulations changed with the Dec 2017 TCJA, and they often are changed. You have to stay current in your field. People are relying on your guidance and your tasks affect them.