First, I'm not aware of any NCAA rule that would make him ineligible. D1 freshman year eligibility is determined by his status as a qualifier. (14.3) However, a college and the conference it belongs to will have eligibility rules, which may include a 2.0 GPA, but it could be less.
If he isn't academically eligible at his first school, he'll have to serve a year in residence as a transfer to a D2 (14.5.5.3.9b) or D3 (14.5.5.1.1). Of course he'd have to sit out a year after transferring to a D1 regardless of his grades.
JC rules vary by sponsoring body, but the NJCAA rule for semester system schools is that he would need 12 hours at a 1.75 during his first semester or sit out 16 weeks. (Article V, sections 10 and 4D/4E). The rule is more complicated for schools on a quarter system.
Staying on the team at his first school may be problematic, depending on how the coaches perceive his chances of succeeding academically in following terms. D1 schools have to meet an Academic Progress Index, and each term a player who doesn't return to school costs a point, as does any player who isn't academically eligible. If the coaches fear that he may have academic issues in the future, they may decide to limit their loss. It is very much in your son's best interests to strive hard to make grades, or at least to show the coaches and athletic department that he has the right attitude.