I have coached/instructed kids for 15+ years in both football (Soccer for those on the other side of the ocean)
and Kyokushin karate (5-16yrs).
Now the fact that you are coaching your own son makes a difference because he will give you **** back like my kids did sometimes.
But i have 1 simple rule
always comes first over quantity.
This is something i have always told all the kids i have coached... if you are going to do something then do it right, take the time you need because once you have this programmed in your head you will never have to think about it again. Why waste time doing 100 bad sit ups when you can do 10 that are 100% perfect. It's always good to tweek what you already have (progression) and this comes over years of working out and will continue the rest of your life, but building a correct foundation will never be replaced. So many people that have worked out themselfs have to more or less start over when they start with a PT or coach because their foundation was wrong from the start and now after all these years they need to fix it.
This is the same methadology i have towards my kids (now 18 and 24) but to all of the kids i have coached/instructed.
If you think like this you won't go wrong