Results are what mater. Any time everybody surfaces in good health the decisions that let to this excellent result are secondary. You can make decisions according to the book and get negative results. Many of the comments here are basic debriefing procedures that air force pilots etc. go through after every mission. We all can learn how to do thing better the next time. Your results were for both you and your son were excellent. You both surfaced healthy and well.
A few debriefing comments. Just an opinion.
1. Safe diving starts on shore. We perform a pre-dive plan and evaluate the dive in all aspects. training, diving group health, knowledge, ability, equipment, dive conditions etc. Dropping someone who has a 3 hour training session to 35 feet is unacceptable in my opinion and something I would not let anyone who I had responsibility for get involved in. While in Cancun Mexico, (I lost my NAUI dive card completed in the 1970s but was certified), I went through the 2 hour training in the pool which consisted of putting on the equipment and to swim around in a 10 foot pool. Next we go out on a boat. They drop a line and say jump in, go down the line and wait on the bottom for the group. I was the first, I dropped to 35 feet and waited for the others. If I had not had previous training years ago I would have been freaked. I watched each person come down. I could tell the ones that had no training. They were freaked. Some could not clear there ears and went up and down the rope like a yo yo. Some could not even get down. I thought none of them were safe to dive. They were on the verge of panic. I burned half my tank waiting for these brittle divers. Bottom line: No certification, not safe.
2. The number one rule in diving " You are responsible for you". First you make sure your safe and then help the fellow diver. You were safe and you helped your son (buddy diver). Note the number one rule. You may have not been aware of it but you were diving alone. If you would have had a problem you had no buddy to help you. You were a solo diver weather you realized it or not. The instructor would have been no help and your son would have been no help. In fact you would have been safer diving alone. The illusion that you had an instructor or fellow buddy looking after you is more dangerous than solo diving where their is no doubt you are on your own. In fact your buddy diver (your son) was a burden on your safety. He took and you gave and he had no ability to reciprocate. Again in my opinion the number one rule in diving is that "You are responsible for you while scuba diving".
3. I have ad some poor experiences with diving instructors and DM's. I have learned rule number one by diving with poor and dangerous instructors and DMs. "You are responsible for you". Don't put your life in some stranger's hands. He is not your daddy. He gets paid poorly and often is involved in repetitive activities that just make you diver number 1794562 on the list that has paid his fee. Some instructors and DM are extremely professional, conscientious and are totally committed to keeping everyone safe and having a good experience. Some are not. You seemed surprised by the instructors answer when he replied that he was aware of your son's air situation. Wen you ask an idiot a question and you get an idiot response it should be no surprise. Common sense seems not so common. You observed the instructor's lack of judgement leaving you both 75 feet behind. You had the common sense to check on your son and bring him to the surface safely. If you would have depended on your dive instructor to even be aware of the situation and to resolve the situation you would not have a son to dive with in the future. Rule number one. "You are responsible for you". Never give that responsibility to anyone else. An excellent buddy diver is an asset, a back up air supply, a helper in an emergency or just to help get the kelp off your tank, and is a redundant total system back up. It is a privilege to have a great dive buddy. A great dive instructor is wonderful asset. But even with an excellent buddy and excellent instructor dive rule number one still applies "you are responsible for you". Taking that responsibility makes you an excellent dive buddy because you are prepared, trained, competent, and stable to help your buddy.
3. Who gives a rats ass what the incompetent instructor thinks. He acted like an idiot and thinks like an idiot. Rule number one. Take full responsibility for yourself while scuba diving. It is good that you did, you may have saved your son's life.
Good heath and good diving