Certainly not an expert, but this is my understanding of how the process works.
Yes, your son needs a release, but in DIII, it is a "self-release". The one-time transfer exemption should still apply so he won't have to sit out a year (at this time. Next year that might not be the case.) I don't have anymore details, I just remember reading about DIII requiring a "self-release" before talking to another school. The NCAA transfer guides will answer your question, and there are probably some others here who have direct experience, perhaps they will chime in.