In summary, the answers to your Ivy questions are that he would have to walk-on after being admitted. The time for your son to receive athletic sponsorship and support from the Coach is past unless there are extenuating circumstances. He would have to apply (late), be admitted (late) and walk-on which is not going to be easy since the Regular Decision deadline was early Jan 2016.
If he has been accepted and the D3 coach wants him & supported him through high academic admissions why are we having this discussion? What is it that he really wants? Your son has good ACT score and GPA. What have other coaches and experts said about his baseball skills and projectability? I'm sensing that he is strong academically but a very late bloomer that has yet to discover if he has any true baseball potential. Some coaches were willing to take a chance on him, but you are wondering if there are other options. Am I off base, if so please let me know. I look forward to your response.