I don't see anything in either the NJCAA Eligibility Rules, or in the D1 Manual that constrains the eligibility of a student who took dual credit classes while in high school, assuming the student didn't play or practice baseball at the JC. Even practicing with the JC team while taking dual credit classes would have minimal impact unless he enrolls at a 4 year school rather than a JC after graduating from high school--that could technically make him a transfer student at the D1.
I don't see any rules that show a problem. Here's a D1 rule that suggests that dual credit courses aren't a problem:
14.2.2.3 Joint College/High School Program. A student-athlete’s eligibility under the five-year rule does not begin while a student is enrolled in a collegiate institution in a joint high school/college academic program for high school students in which the courses count as both high school graduation credit and college credit, provided the student has not officially graduated from high school and does not participate in intercollegiate athletics while enrolled in the joint program.
Of course that just covers the D1 5 year rule. But I don't see anything regarding transfers or intial eligibility, etc.