Dad wants no part of helping our out of control 3 year old who hurts his sister.

Hi mums. I've asked for advice previously regarding my 3 year old and the general consensus was he needed attention or was just a normal 3 year old.
Well after referrals from paed and health nurse (based on their observations, not my suggestion) we had our first psychologist visit today and unfortunately he has some issues we will work on over the next few visits.
My problem is I don't live with their dad and when I phoned him today to tell him about the appointment he ridiculed me. He said I was crazy for taking him and following advice of professionals. His mum had raised concerns with our sons behaviour and I asked him to tell her and he told me I need my head read.
Their dad is very immature and frankly naive of life in the real world, mollycoddled by his mum and family.
He has our son every weekend, our daughter is 19 months and has the occasional visit like when I'm called into work.
I feel like I'm fighting a no win battle, his dad may be part of the problem and have to play apart in the treatment but is just not interested. He's refused point blank to come to the psychology sessions.