You are right on target when you say that one-on-one learning is very different from classroom learning, that more can be accomplished in a shorter amount of time! That is one reason that, when a student completes a text for a full-year course such as for geometry, his course work for that subject is considered complete, even if he hasn't yet reached 180 hours.
That would also follow for a course of study that you have set out for him for the year. If he has completed all the work that you have assigned him for a year-long class, he receives a full credit for that class regardless of how many hours he has put in. [Remember that each full-year class in your homeschool is worth one credit; a minimum of 22 credits is required for graduation.]
Certainly, as you say, he doesn't have to spend an hour at a desk for each subject, just as if he were in public school. [In fact, it sounds to me as if you might be interested in the approaches offered in the High School of Your Dreams program, which is a much more results-oriented approach.] And your son doesn't receive credited hours in geometry, for example, for only the time he spends with the tutor. He also receives credit hours for the times spent on any and all work that he does in that subject. [High School of Your Dreams Guidebook goes into this at length.]
The primary things to remember are that 1. if you should at some point decide to get a diploma from Clonlara School or North Atlantic Regional High School, your son will need those documented hours and some sort of portfolio of work to meet their credit requirements and 2. if you don't have a certified diploma, colleges will likely put more emphasis on SAT and ACT scores to make sure he is prepared for college. College admissions might also ask for documentation or a portfolio of his work as well. [Keeping a list of texts used and completed is a good idea, too.]
So while you are absolutely right that boxing him in to a replica of public school defeats the purpose of homeschooling, it is important still to find a balance between too much and too little so he won't be at a disadvantage for college admissions or even for the work force, just as you mentioned.
It sounds to me as if you may already have a pretty good idea of what will be a year's course work for each of his subjects? If so, and he completes the course as you have planned it, then he should receive a year's credit for a year of work.
I hope that makes sense! If not, feel free to contact me!
God bless and guide you as you homeschool for eternity,