My S has a 4.3 weighted GPA and a 35 ACT. Participates in year round sports and volunteers in the summer. What are his chances of getting into an ivy league school?
More information is necessary.
Generally speaking, off of that abbreviated thumbnail sketch you just provided, as good as most (which is about 5% - 10%, depending on the Ivy League schools to which he applies) and better than many (which is closed to 5%).