My DS attends a high school with a similar set-up. He is a senior and will be graduating with 50-60 university credits under his belt. In researching and applying to colleges he's found that how his previous college classes are viewed depends greatly on the policy of the college and whether he wants to receive credit or placement for the classes he's already taken. So, as you look toward high school with your DS there are a number of things to take into consideration.
First, will your son's credits be earned through dual enrollment classes (that is, will he earn both HS and college credit for them)? If he does not, graduating with 50-60 college credits will probably classify him as a transfer student, not a freshman. If that's the case, each college he is applying to will look at his transcript and decide which of his credits it will accept. It's not really a problem, if he's okay with skipping the first year or so of college classes. Also keep in mind that that there are usually less transfer spots available and often there aren't as many readily available scholarship opportunities.
On the other hand, if they are going to be dual enrollment credits, there'll be no worries, because they won't count as college credits for admissions purposes (and he'll still be considered a freshman after HS graduation). In such a scenario, the college credits your DS earns during high school will most likely be counted for placement not for credit. So if he's already taken English 101, he won't have to take it again when he gets to college, but he won't get to count the 3 credits he earned from taking it during high school towards his college degree requirements either.
There are schools that will grant credit, as well as placement, for dual enrollment classes, but each school has its own policy. For the most part, though, the Ivies do not accept dual enrollment credits (or AP/IB credits, for that matter). At less competitive schools, he will probably find that a number of his credits will transfer (though each school usually does have a limit as to how many credits you can bring with you, so he might find that he can't bring all of them).
Either way I don't really see the credits as being a problem. If he does well in college classes during high school all it will do is prove that he's capable of succeeding in college-level courses in a college atmosphere, which is one of the things the admissions folks are looking for. Of course, if he has his heart set on bringing all those credits with him when he goes off to college, his best bet will be at the university at which they were earned.
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