I was also going to mention the rec. letter from the professor.
DS audited Calc 1 over the summer. This was between 8 and 9 grade. Since the University class started in the beginning of May, I took him from school a good month early. He really enjoyed that!
He audited the class but took all the tests, did all the homework etc. The professor offered to write a letter should we ever need it (disclosure - we know a lot of math prof. at that particular U).
The reason for the class was so he could take AP Calc BC as a freshman, which he is now doing. Next year he will have to take classes at the U, but his HS allows it early - there is actually one freshman that is taking Calc 3 now.
The biggest difference between the college and HS courses is the fact that you do them in half the time and while the material might not be overwhelming, the workload might.
If you are looking at the early graduation from HS your son's situation will be looked upon more carefully and you will be able to explain more than regular college applicant. Ask professors to write a rec. letters that will include all of his grades etc. This to me is a win/win situation becasue if the grade is good, you ask for the letter if it is not, you do not.
