cheech this sounds very similar to the route I took to become a coach. I was dead set on becoming a physical therapist after I hurt my knee in high school and had to take PT. I thought it was the coolest job anyone could have and it paid well. I started college and realized pretty quickly I wasn't a disciplined enough student to go that route (especially playing baseball). So I decided to go into something else but not sure what. My second year I started coaching at a local high school a college team mate was named head coach at. This is where I wanted to become a coach at - working with them.
Anyway back to your son and I think it's great he is going into this line of work.
I think there are two ways to get into college coaching.
1. Pretty much the route you described. Get teaching job and work way up. Only problem with it is it's not a sure thing and it hurts your resume applying for college jobs when it says "high school coach". There is a ton of competition for open jobs and you really need to stand out. This is the route I have pretty much taken and I have found it very difficult to find a job. I have had offers but they were for like $5000 and that is it. After teaching 10 years I cannot afford to make a jump like that financially.
2. Have your son finish his playing days and get a graduate asst. coaching position. This way he can work on his masters degree (most colleges look for a PE masters) and get on his resume college coaching. Once again he won't be making a lot of money but at least he will have his foot in the door and making contacts.
Being a young college graduate I think his best bet is route 2 because he will be poor coming out college so it won't be a shock to his pocket being poor getting started out in this line of profession.
Best of luck to him and tell him it will be a very rewarding (but tough) line of work.
When life hands you gators - make Gatorade