Your original question was: "What do I need to do to ensure I am involved in son's life and decisions surrounding his care?"
I suggest that you consult a local attorney to see if you have any options. Is it possible that son received some sort of settlement as a result of the accident and Dad is using those funds?
Son has been in a coma then a vegetative state since the accident. He does not walk, talk or communicate.
I would have to assume that he needs a legal guardian and not sure that Dad has the right to just step in and "appoint" himself as that person. If all Dad has is a medical POA and is using son's funds outside of the law, then someone needs to step up and take a look at the situation. Contact an attorney and lay it out for them.
"Never argue with stupid people, they will drag you down to their level and then beat you with experience." - Mark Twain