Just spitballing some thoughts here:
Has it been your son reaching out to the advisor? It would be SOP for an advisor to not respond to a parent.
Have you seen the correspondence between your son and the faculty member regarding the incomplete? When I have a student that needs an incomplete, I leave a message in the master advising site that explicitly states what needs to be done, and what the timeline should/needs to be.
Do you know if the faculty is actively ignoring your son, or are they truely unavailable? (Are they alive?) The Engineering Dean should know this, or at least have a feel. Is the Dean surprised by this behavior? Is the faculty member under contract now? If the faculty member is not responding because they are on a 10 month contract, they should bave made that clear when communicating the timeline requirements.
If there are research-based grad students in the department, they might have a feel for what is going on. Is there a TA from a different course your son can reach out to so you can figure out what is happening.
I am sorry.