3 yr. old refuses to use potty

I have a 3 and a half year old nephew who refuses to even sit on the potty for his parents.  But he will sit on it with me.  And neither his mom or dad can get him in a pair of underwear.  But again he will wear them for me as long as I let him wear a pair of shoes.  When his mom tries to get him to sit on the potty he throws a huge fit!  He bangs his head on the floor and becomes violent towards his mom.  I am trying to help them the best that I can, but they don't follow through with the things that I have done with him because of his violent behaviors.  The bad behavior becomes even worse when she tries to get him in a pair of underwear.  And because (at the time) I only watched him a couple hours a day I was not successful with actually getting to go on the potty but he was becoming more comfortable with it.  So I guess I am wondering how do I get the little boy and his parents on the same page?  I have been asked to babysit again for a couple of months and I don't know if I should do it or if he should go to daycare.  I don't know who will be successful at getting him potty trained.  I want him to advance and be able to be a normal three year old instead of a child that is stuck at home because his parents are embarrassed of him!